{"id":684,"date":"2018-08-18T05:13:14","date_gmt":"2018-08-18T05:13:14","guid":{"rendered":"https:\/\/cbselibrary.com\/?p=684"},"modified":"2018-08-18T09:07:27","modified_gmt":"2018-08-18T09:07:27","slug":"similarity-of-triangles","status":"publish","type":"post","link":"https:\/\/cbselibrary.com\/similarity-of-triangles\/","title":{"rendered":"Criteria For Similarity Of Triangles"},"content":{"rendered":"
AAA similarity criterion:<\/strong> If in two triangles, corresponding angles are equal, then the triangles are similar. Read More:<\/b><\/p>\n Example 1: \u00a0 \u00a0<\/strong>In figure, find \u2220L. Example 2: \u00a0 \u00a0<\/strong>Examine each pair of triangles in Figure, and state which pair of triangles are similar. Also, state the\u00a0similarity criterion used by you for answering the question and write the similarity relation in symbolic form. Example 3: \u00a0 \u00a0<\/strong>In figure, QA and PB are perpendicular to AB. If AO = 10 cm, BO = 6 cm and PB = 9 cm. Find AQ. Example 4: \u00a0 \u00a0\u00a0<\/strong>In figure, \u2206ACB ~ \u2206APQ. If BC = 8 cm, PQ = 4 cm, BA = 6.5 cm, AP = 2.8 cm, find CA and AQ. Example 5: \u00a0 \u00a0\u00a0<\/strong>The perimeters of two similar triangles ABC and PQR are respectively 36 cm and 24 cm. If PQ = 10 cm, find AB. Example 6: \u00a0 \u00a0\u00a0<\/strong>In figure, \u2220CAB = 90\u00ba and AD \u22a5\u00a0BC. If AC = 75 cm, AB = 1 m and BD = 1.25 m, find AD. Example 7: \u00a0 \u00a0\u00a0<\/strong>In figure, if \u2220A = \u2220C, then prove that \u2206AOB ~ \u2206COD. Example 8: \u00a0 \u00a0\u00a0<\/strong>In figure, \u00a0\\( \\frac{AO}{OC}=\\frac{BO}{OD}=\\frac{1}{2} \\) \u00a0 and AB = 5 cm. Find the value of DC. Example 9: \u00a0 \u00a0\u00a0<\/strong>In figure, considering triangles BEP and CPD, prove that BP \u00d7 PD = EP \u00d7 PC. Example 10: \u00a0 \u00a0\u00a0<\/strong>D is a point on the side BC of \u2206ABC such that \u2220ADC = \u2220BAC. Example 11: \u00a0 \u00a0\u00a0<\/strong>P and Q are points on sides AB and AC respectively of \u2206ABC. If AP = 3 cm, PB = 6cm. AQ = 5 cm and QC = 10 cm, show that BC = 3PQ. Example 12: \u00a0 \u00a0<\/strong>\u00a0In figure, \u2220A = \u2220CED, prove that \u2206CAB ~ \u2206CED. Also, find the value of x. Example 13: \u00a0 \u00a0<\/strong>\u00a0In the figure, E is a point on side CB produced of an isosceles \u2206ABC with AB = AC. If AD \u22a5\u00a0BC and EF \u22a5\u00a0AC, prove that \u2206ABD ~ \u2206ECF. Example 14: \u00a0 \u00a0<\/strong>In figure, \u2220BAC = 90\u00ba and segment AD\u00a0\u22a5 BC. Prove that AD2<\/sup> = BD \u00d7 DC. Example 15: \u00a0 \u00a0<\/strong>In an isosceles \u2206ABC, the base AB is produced both ways in P and Q such that AP \u00d7 BQ = AC2<\/sup>\u00a0\u00a0and CE are the altitudes. Prove that \u2206ACP ~ \u2206BCQ. Example 16: \u00a0 \u00a0\u00a0<\/strong>The diagonal BD of a parallelogram ABCD intersects the segment AE at the point F,\u00a0where E is any point on the side BC. Prove that DF \u00d7 EF = FB \u00d7 FA. Example 17: \u00a0 \u00a0<\/strong>Through the mid-point M of the side CD of a parallelogram ABCD, the line BM is drawn intersecting AC in L and AD produced in E. Prove that EL = 2 BL. Example 17: \u00a0 \u00a0\u00a0<\/strong>In figure, ABCD is a trapezium with AB || DC. If \u2206AED is similar to \u2206BEC, prove that AD = BC. Example 18: \u00a0 \u00a0\u00a0<\/strong>A vertical stick 20 cm long casts a shadow 6 cm long on the ground. At the same time, a tower casts a shadow 15 m long on the ground. Find the height of the tower. Example 19: \u00a0 \u00a0\u00a0<\/strong>If a perpendicular is drawn from the vertex containing the right angle of a right triangle to the hypotenuse then prove that the triangle on each side of the perpendicular are similar to each other and to the original triangle. Also, prove that the square of the perpendicular is equal to the product of the lengths of the two parts of the hypotenuse. Example 20: \u00a0 \u00a0\u00a0<\/strong>Prove that the line segments joining the mid points of the sides of a triangle form four triangles, each of which is similar to the original triangle. Example 21: \u00a0 \u00a0\u00a0<\/strong>In \u2206ABC, DE is parallel to base BC, with D on AB and E on AC. If \u00a0 \\(\\frac{AD}{DB}=\\frac{2}{3}\\) , find \\(\\frac{BC}{DE}\\). Criteria For Similarity Of Triangles AAA similarity criterion: If in two triangles, corresponding angles are equal, then the triangles are similar. AA Similarity criterion: If in two triangles, two angles of one triangle are respectively equal the two angles of the other triangle, then the two triangles are similar. SSS Similarity criterion: If in two … Read more<\/a><\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"spay_email":""},"categories":[5],"tags":[76,77,71],"yoast_head":"\n
\nAA Similarity criterion:<\/strong> If in two triangles, two angles of one triangle are respectively equal the two angles of the other triangle, then the two triangles are similar.
\nSSS Similarity criterion:<\/strong> If in two triangles, corresponding sides are in the same ratio, then the two triangles are similar.
\nSAS similarity criterion:<\/strong>\u00a0If one angle of a triangle is equal to one angle of the other triangle and the sides including these angles are proportional, then the triangles are similar.
\nEquiangular Triangles:<\/strong>
\nTwo triangles are said to be equiangular, if their corresponding angles are equal.
\nIf two triangles are equiangular, then they are similar.
\nTwo triangles ABC and DEF such that
\n\u2220A = \u2220D, \u2220B = \u2220E and \u2220C = \u2220F.
\nThen \u2206ABC ~ \u2206DEF and
\n\\(\\frac{AB}{DE}=\\frac{BC}{EF}=\\frac{AC}{DF}\\)
\n<\/p>\n\n
Criteria For Similarity Of Triangles With Examples<\/strong><\/h2>\n
\n
\nSol. \u00a0 \u00a0<\/strong>In \u2206ABC and \u2206LMN,
\n\\( \\frac{AB}{LM}=\\frac{4.4}{11}=\\frac{2}{5}\\)
\n\\( \\frac{BC}{MN}=\\frac{4}{10}=\\frac{2}{5} \\)
\n\\( \\frac{CA}{NL}=\\frac{3.6}{9}=\\frac{2}{5} \\)
\n\\( \\Rightarrow \\frac{AB}{LM}=\\frac{BC}{MN}=\\frac{CA}{NL} \\)
\n\u2206ABC ~ \u2206LMN (SSS similarity)
\n\u2220L = \u2220A
\n= 180\u00ba \u2013 \u2220B \u2013 \u2220C
\n= 180\u00ba \u2013 50\u00ba \u2013 70\u00ba = 60\u00ba
\n\u2220L = 60\u00ba<\/p>\n
\n
\nFigure (i)
\n
\nFigure (ii)
\n
\nFigure (iii)
\n
\nFigure (iv)
\n
\nFigure (v)
\n
\nFigure (vi)
\n
\nFigure (vii)
\nSol. \u00a0 \u00a0\u00a0(i)<\/strong>\u00a0\u2220A = \u2220Q, \u2220B = \u2220P and \u2220C = \u2220R.
\n\u2234\u00a0\u2206ABC ~ \u2206QPR \u00a0 \u00a0 \u00a0 \u00a0 \u00a0(AAA-similarity)
\n(ii)<\/strong> In triangle PQR and DEF, we observe that
\n\\(\\frac{PQ}{DE}=\\frac{QR}{EF}=\\frac{PR}{DF}=\\frac{1}{2} \\)
\nTherefore, by SSS-criterion of similarity, we have
\n\u2206PQR ~ \u2206DEF
\n(iii)<\/strong> SAS-similarity is not satisfied as included angles are not equal.
\n(iv)<\/strong>\u00a0\u2206CAB ~ \u2206QRP \u00a0 \u00a0 \u00a0 \u00a0 (SAS-similarity), as
\n\\(\\frac{CA}{QR}=\\frac{CB}{QP} \\) \u00a0 and \u2220C = \u2220Q.
\n(v)<\/strong> In \u2206\u2019s ABC and DEF, we have
\n\u2220A = \u2220D = 80\u00ba
\n\\(\\text{But, }\\frac{AB}{DE}\\ne \\frac{AC}{DF} \\) \u00a0 \u00a0 \u00a0 \u00a0[\u2235\u00a0 AC is not given]
\nSo, by SAS-criterion of similarity these two triangles are not similar.
\n(vi)<\/strong> In \u2206\u2019s DEF and MNP, we have
\n\u2220D = \u2220M = 70\u00ba
\n\u2220E = \u2220N = 80\u00ba \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [\u2235\u00a0 \u2220N = 180\u00ba \u2013 \u2220M \u2013 \u2220P = 180\u00ba \u2013 70\u00ba \u2013 30\u00ba = 80\u00ba]
\nSo, by AA-criterion of similarity
\n\u2206DEF ~ \u2206MNP.
\n(vii)<\/strong> FE = 2 cm, FD = 3 cm, ED = 2.5 cm
\nPQ = 4 cm, PR = 6 cm, QR = 5 cm
\n\u2234\u00a0\u2206FED ~ \u2206PQR \u00a0 \u00a0 \u00a0 \u00a0(SSS-similarity)<\/p>\n
\nSol. \u00a0 \u00a0\u00a0<\/strong>In triangles AOQ and BOP, we have
\n\u2220OAQ = \u2220OBP \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Each equal to 90\u00ba]
\n\u2220AOQ = \u2220BOP \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Vertically opposite angles]
\nTherefore, by AA-criterion of similarity
\n\u2206AOQ ~ \u2206BOP
\n
\n\\( \\Rightarrow \\frac{AO}{BO}=\\frac{OQ}{OP}=\\frac{AO}{BP} \\)
\n\\( \\Rightarrow \\frac{AO}{BO}=\\frac{AQ}{BP}\\Rightarrow \\frac{10}{6}=\\frac{AQ}{9} \\)
\n\\( \\Rightarrow AQ=\\frac{10\\times 9}{6}=15\\text{ }cm \\)<\/p>\n
\nSol.<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0 We have, \u2206ACB ~ \u2206APQ
\n
\n\\( \\Rightarrow \\frac{AC}{AP}=\\frac{CB}{PQ}=\\frac{AB}{AQ} \u00a0\\)
\n\\( \\Rightarrow \\frac{AC}{AP}=\\frac{CB}{PQ}\\text{\u00a0 and\u00a0\u00a0 }\\frac{CB}{PQ}=\\frac{AB}{AQ} \\)
\n\\( \\Rightarrow \\frac{AC}{2.8}=\\frac{8}{4}\\text{\u00a0 and\u00a0\u00a0 }\\frac{8}{4}=\\frac{6.5}{AQ} \u00a0\\)
\n\\( \\Rightarrow \\frac{AC}{2.8}=2\\text{\u00a0\u00a0 and\u00a0\u00a0 }\\frac{6.5}{AQ}=2 \\)
\nAC = (2 \u00d7 2.8) cm = 5.6 cm and \u00a0 \u00a0\\( AQ=\\frac{6.5}{2}\\text{ }cm\\text{ }=\\text{ }3.25\\text{ }cm \\)<\/p>\n
\nSol.<\/strong>\u00a0\u00a0\u00a0\u00a0\u00a0Since the ratio of the corresponding sides of similar triangles is same as the ratio of their perimeters.
\n\u2234\u00a0\u2206ABC ~ \u2206PQR
\n\\( \\Rightarrow \\frac{AB}{PQ}=\\frac{BC}{QR}=\\frac{AC}{PR}=\\frac{36}{24} \\)
\n\\( \\Rightarrow \\frac{AB}{PQ}=\\frac{36}{24}\\Rightarrow \\frac{AB}{10}=\\frac{36}{24} \\)
\n\\( \\Rightarrow AB=\\frac{36\\times 10}{24}cm\\text{ }=\\text{ }15\\text{ }cm \\)<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0\u00a0We have,
\nAB = 1 m = 100 cm, AC = 75 cm and BD = 125 cm
\nIn \u2206BAC and \u2206BDA, we have
\n\u2220BAC = \u2220BDA \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Each equal to 90\u00ba]
\nand, \u2220B = \u2220B
\nSo, by AA-criterion of similarity, we have
\n\u2206BAC ~ \u2206BDA
\n
\n\\( \\Rightarrow \\frac{BA}{BD}=\\frac{AC}{AD} \\)
\n\\( \\Rightarrow \\frac{100}{125}=\\frac{75}{AD} \\)
\n\\( \\Rightarrow AD=\\frac{125\\times 75}{100}cm\\text{ }=\\text{ }93.75\\text{ }cm \\)<\/p>\n
\n
\nSol.<\/strong>\u00a0 \u00a0\u00a0In triangles AOB and COD, we have
\n\u2220A = \u2220C \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [Given]
\nand, \u22201 = \u22202 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Vertically opposite angles]
\nTherefore, by AA-criterion of similarity, we have
\n\u2206AOB ~ \u2206COD<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 In \u2206AOB and \u2206COD, we have
\n\u2206AOB = \u2206COD \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Vertically opposite angles]
\n\\( \\frac{AO}{OC}=\\frac{BO}{OD} \\)\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [Given]
\n
\nSo, by SAS-criterion of similarity, we have
\n\u2206AOB ~ \u2206COD
\n\\( \\Rightarrow \\frac{AO}{OC}=\\frac{BO}{OD}=\\frac{AB}{DC} \\)
\n\\( \\Rightarrow \\frac{1}{2}=\\frac{5}{DC} \\) \u00a0 \u00a0 \u00a0 \u00a0[\u2235\u00a0 \u00a0AB = 5 cm]
\n\u21d2 DC = 10 cm<\/p>\n
\nSol. \u00a0 \u00a0Given:<\/strong> A \u2206ABC in which BD \u22a5\u00a0AC and CE \u22a5\u00a0AB and BD and CE intersect at P.
\nTo Prove:<\/strong> BP \u00d7 PD = EP \u00d7 PC
\nProof:<\/strong> In \u2206EPB and \u2206DPC, we have
\n\u2220PEB = \u2220PDC \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [Each equal to 90\u00ba]
\n\u2220EPB = \u2220DPC \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [Vertically opposite angles]
\n
\nThus, by AA-criterion of similarity, we have
\n\u2206EPB ~ \u2206DPC
\n\\(\\frac{EP}{DP}=\\frac{PB}{PC}\\)
\n\u21d2 BP \u00d7 PD = EP \u00d7 PC<\/p>\n
\nProve that \u00a0\\(\\frac{CA}{CD}=\\frac{CB}{CA}\\) \u00a0 or, CA2<\/sup> = CB \u00d7 CD.
\nSol. \u00a0\u00a0<\/strong> In \u2206ABC and \u2206DAC, we have
\n\u2220ADC = \u2220BAC and \u2220C = \u2220C
\n
\nTherefore, by AA-criterion of similarity, we have
\n\u2206ABC ~ \u2206DAC
\n\\( \\Rightarrow \\frac{AB}{DA}=\\frac{BC}{AC}=\\frac{AC}{DC} \\)
\n\\( \\Rightarrow \\frac{CB}{CA}=\\frac{CA}{CD} \\)<\/p>\n
\nSol. \u00a0\u00a0<\/strong> We have,
\nAB = AP + PB = (3 + 6) cm = 9 cm
\nand, AC = AQ + QC = (5 + 10) cm = 15 cm.
\n\\( \\therefore \\frac{AP}{AB}=\\frac{3}{9}=\\frac{1}{3}\\text{ and }\\frac{AQ}{AC}=\\frac{5}{15}=\\frac{1}{3} \\)
\n\\( \\Rightarrow \\frac{AP}{AB}=\\frac{AQ}{AC} \\)
\n
\nThus, in triangles APQ and ABC, we have
\n\\( \\frac{AP}{AB}=\\frac{AQ}{AC} \\) \u00a0 \u00a0and \u2220A = \u2220A \u00a0 \u00a0 \u00a0 \u00a0[Common]
\nTherefore, by SAS-criterion of similarity, we have
\n\u2206APQ ~ \u2206ABC
\n\\( \\Rightarrow \\frac{AP}{AB}=\\frac{PQ}{BC}=\\frac{AQ}{AC} \\)
\n\\( \\Rightarrow \\frac{PQ}{BC}=\\frac{AQ}{AC}\\Rightarrow \\frac{PQ}{BC}=\\frac{5}{15} \\)
\n\\( \\Rightarrow \\frac{PQ}{BC}=\\frac{1}{3} \\)
\n\u21d2 BC = 3PQ<\/p>\n
\n
\nSol.<\/strong>\u00a0 \u00a0 \u00a0In \u2206CAB and \u2206CED, we have
\n\u2220A = \u2220CED and \u2220C = \u2220C \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [common]
\n\u2206CAB ~ \u2206CED
\n\\( \\Rightarrow \\frac{CA}{CE}=\\frac{AB}{DE}=\\frac{CB}{CD} \\)
\n\\( \\Rightarrow \\frac{AB}{DE}=\\frac{CB}{CD}\\Rightarrow \\frac{9}{x}=\\frac{10+2}{8} \\)
\n\u21d2 \u00a0x = 6 cm<\/p>\n
\n
\nSol.<\/strong>\u00a0 \u00a0 \u00a0Given:<\/strong> A \u2206ABC in which AB = AC and AD \u22a5\u00a0BC. Side CB is produced to E and EF \u22a5\u00a0AC.
\nTo prove:<\/strong>\u00a0\u2206ABD ~ \u2206ECF.
\nProof:<\/strong> we known that the angles opposite to equal sides of a triangle are equal.
\n\u2220B = \u2220C [\u2235\u00a0 AB = AC]
\nNow, in \u2206ABD and \u2206ECF, we have
\n\u2234 \u00a0\u2220B = \u2220C \u00a0 \u00a0 \u00a0 \u00a0[proved above]
\n\u2220ADB = \u2220EFC = 90\u00ba
\n\u2234 \u00a0\u2206ABD ~ \u2206ECF \u00a0 \u00a0 \u00a0 [By AA-similarity]<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 \u00a0In \u2206ABD and \u2206ACD, we have
\n
\n\u2220ADB = \u2220ADC \u00a0 \u00a0 \u00a0 [Each equal to 90\u00ba]
\nand, \u2220DBA = \u2220DAC
\n\\(\\left[ Each\\ equal\\ to\\ complement\\ of\\angle BAD\\ i.e.,\\ {{90}^{\\text{o}}}\\ -\\angle BAD \\right] \\)
\nTherefore, by AA-criterion of similarity, we have
\n\u2206DBA ~ \u2206DAC
\n\\(\\left[ \\therefore \\ \\ \\angle D\\leftrightarrow \\ \\angle D,\\ \\angle DBA\\leftrightarrow \\ \\angle DAC\\ and\\ \\angle BAD\\leftrightarrow \\ \\angle DCA \\right] \\)
\n\\(\\Rightarrow \\frac{DB}{DA}=\\frac{DA}{DC} \\)
\n\\(\\left[ In\\ similar\\ triangles\\ corresponding\\text{ }sides\\ are\\ proportional \\right] \\)
\n\\(\\Rightarrow \\frac{BD}{AD}=\\frac{AD}{DC}\u00a0\\)
\nAD2<\/sup>\u00a0= BD \u00d7 DC<\/p>\n
\nSol. \u00a0\u00a0<\/strong> CA = CB \u00a0\u21d2\u00a0\u2220CAB = \u2220CBA
\n\u21d2 180\u00ba \u2013 \u2220CAB = 180\u00ba \u2013 \u2220CBA
\n\u21d2 \u2220CAP = \u2220CBQ
\nNow, AP \u00d7 BQ = AC2<\/sup>
\n\\( \\Rightarrow \\frac{AP}{AC}=\\frac{AC}{BQ}\\Rightarrow \\frac{AP}{AC}=\\frac{BC}{BQ}\\) \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [\u2235\u00a0 AC = BC]
\n
\nThus, \u2220CAP = \u2220CBQ \u00a0 \u00a0and \u00a0 \u00a0 \u00a0 \u00a0 \\( \\frac{AP}{AC}=\\frac{BC}{BQ} \\)
\n\u2234 \u00a0\u2206ACP ~ \u2206BCQ.<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 In \u2206AFD and \u2206BFE, we have
\n\u22201 = \u22202 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Vertically opposite angles]
\n\u22203 = \u22204 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Alternate angles]
\n
\nSo, by AA-criterion of similarity, we have
\n\u2206FBE ~ \u2206FDA
\n\\( \\Rightarrow \\frac{FB}{FD}=\\frac{FD}{FA} \u00a0\\)
\n\\( \\Rightarrow \\frac{FB}{DF}=\\frac{EF}{FA} \\)
\n\u21d2 DF \u00d7 EF = FB \u00d7 FA<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 In \u2206BMC and \u2206EMD, we have
\nMC = MD \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [\u2235 M is the mid-point of CD]
\n\u2220CMB = \u2220EMD \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Vertically opposite angles]
\nand, \u2220MBC = \u2220MED \u00a0 \u00a0 \u00a0 \u00a0 [Alternate angles]
\nSo, by AAS-criterion of congruence, we have
\n\u2234 \u2206BMC \u2245\u00a0\u2206EMD
\n\u21d2\u00a0BC = DE \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 ….(i)
\nAlso, AD = BC \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0….(ii)
\n[\u2235 ABCD is a parallelogram]
\nAD + DE = BC + BC
\n\u21d2\u00a0\u00a0AE = 2 BC \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 ….(iii)
\n
\nNow, in \u2206AEL and \u2206CBL, we have
\n\u2220ALE = \u2220CLB \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Vertically opposite angles]
\n\u2220EAL = \u2220BCL \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Alternate angles]
\nSo, by AA-criterion of similarity of triangles,
\nwe have
\n\u2206AEL ~ \u2206CBL
\n\\( \\Rightarrow \\frac{EL}{BL}=\\frac{AE}{CB}\\Rightarrow \\frac{EL}{BL}=\\frac{2BC}{BC}\\) \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Using equations (iii)]
\n\\( \\Rightarrow \\frac{EL}{BL}=2 \\)
\n\u21d2\u00a0\u00a0EL = 2BL<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 In \u2206EDC and \u2206EBA, we have
\n\u22201 = \u22202 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Alternate angles]
\n\u22203 = \u22204 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Alternate angles]
\nand, \u2220CED =\u00a0\u2220AEB \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Vertically opposite angles]
\n\u2234 \u00a0\u2206EDC ~ \u2206EBA
\n
\n\\( \\Rightarrow \\frac{ED}{EB}=\\frac{EC}{EA} \\)
\n\\( \\Rightarrow \\frac{ED}{EB}=\\frac{EB}{EA} \\)\u00a0 \u00a0 \u00a0….(i)
\nIt is given that \u2206AED ~ \u2206BEC
\n\\( \\Rightarrow \\frac{ED}{EB}=\\frac{EA}{EB}=\\frac{AD}{BC} \\) \u00a0 \u00a0 \u00a0….(ii)
\nFrom (i) and (ii), we get
\n\\( \\frac{EB}{EA}=\\frac{EA}{EB} \\)
\n\u21d2\u00a0\u00a0\u00a0(EB)2<\/sup> = (EA)2<\/sup>
\n\u21d2\u00a0\u00a0\u00a0EB = EA
\nSubstituting EB = EA in (ii), we get
\n\\(\\frac{EA}{EA}=\\frac{AD}{BC}\\Rightarrow \\frac{AD}{BC}=1\\)
\n\u21d2 \u00a0 AD = BC<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 Let the sun\u2019s altitude at that moment be \u03b8.
\n\u2206PQM ~ \u2206ABC
\n
\n\\( \\Rightarrow \\frac{MP}{MQ}=\\frac{AC}{CB} \\)
\n\\( \\Rightarrow \\frac{h}{15}=\\frac{20}{6} \\)
\n\u2234 \u00a0Height of the tower = 50 m.<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 Given:<\/strong> A right triangle ABC right angled at B, BD \u22a5\u00a0AC.
\nTo Prove:<\/strong>
\n(i) \u2206ADB ~ \u2206BDC \u00a0 \u00a0 \u00a0 (ii) \u2206ADB ~ \u2206ABC
\n(iii) \u2206BDC ~ \u2206ABC \u00a0 \u00a0(iv) BD2<\/sup> = AD \u00d7 DC
\n(v) AB2<\/sup>= AD \u00d7 AC \u00a0 \u00a0 \u00a0(vi) BC2<\/sup> = CD \u00d7 AC
\nProof:<\/strong>
\n
\n(i) \u00a0<\/strong>We have,
\n\u2220ABD + \u2220DBC = 90\u00ba
\nAlso, \u2220C + \u2220DBC + \u2220BDC = 180\u00ba
\n\u21d2\u00a0\u00a0\u00a0\u2220C + \u2220DBC + 90\u00ba = 180\u00ba
\n\u21d2\u00a0\u00a0\u00a0\u2220C + \u2220DBC = 90\u00ba
\nBut, \u2220ABD + \u2220DBC = 90\u00ba
\n\u2234 \u2220ABD + \u2220DBC = \u2220C + \u2220DBC
\n\u21d2\u00a0\u00a0\u2220ABD = \u2220C \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0….(i)
\nThus, in \u2206ADB and \u2206BDC, we have
\n\u2220ABD = \u2220C \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [From (i)]
\nand, \u2220ADB = \u2220BDC \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [Each equal to 90\u00ba]
\nSo, by AA-similarity criterion, we have
\n\u2206ADB ~ \u2206BDC
\n(ii) \u00a0<\/strong>In \u2206ADB and \u2206ABC, we have
\n\u2220ADB = \u2220ABC \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Each equal to 90\u00ba]
\nand, \u2220A = \u2220A \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Common]
\nSo, by AA-similarity criterion, we have
\n\u2206ADB ~ \u2206ABC
\n(iii) \u00a0<\/strong>In \u2206BDC and \u2206ABC, we have
\n\u2220BDC = \u2220ABC \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Each equal to 90\u00ba]
\n\u2220C = \u2220C \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0[Common]
\nSo, by AA-similarity criterion, we have
\n\u2206BDC ~ \u2206ABC
\n(iv) \u00a0<\/strong>From (i), we have
\n\u2206ADB ~ \u2206BDC
\n\\( \\Rightarrow \\frac{AD}{BD}=\\frac{BD}{DC}\\)
\n\u21d2 BD2<\/sup> = AD \u00d7 DC
\n(v) \u00a0<\/strong>From (ii), we have
\n\\( \\Rightarrow \\frac{AD}{AB}=\\frac{AB}{AC}\\)
\n\u2206ADB ~ \u2206ABC
\n\u21d2 AB2<\/sup> = AD \u00d7 AC
\n(vi) \u00a0<\/strong>From (iii), we have
\n\\( \\Rightarrow \\frac{BC}{AC}=\\frac{DC}{BC}\\)
\n\u2206BDC ~ \u2206ABC
\n\u21d2 BC2<\/sup> = CD \u00d7 AC<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 \u00a0Given:<\/strong>\u00a0\u2206ABC in which D, E, F are the
\nmid-points of sides BC, CA and AB respectively.
\nTo Prove:<\/strong> Each of the triangles AFE, FBD, EDC and DEF is similar to \u2206ABC.
\nProof:<\/strong> Consider triangles AFE and ABC.
\nSince F and E are mid-points of AB and AC respectively.
\n\u2234 \u00a0FE || BC
\n\u21d2 \u2220AEF = \u2220B \u00a0 \u00a0 \u00a0\u00a0[Corresponding angles]
\nThus, in \u2206AFE and \u2206ABC, we have
\n\u2220AFE = \u2220B
\nand, \u2220A = \u2220A \u00a0 \u00a0 \u00a0 \u00a0[Common]
\n\u2234 \u00a0\u2206AFE ~ \u2206ABC.
\n
\nSimilarly, we have
\n\u2206FBD ~ \u2206ABC and \u2206EDC ~ \u2206ABC.
\nNow, we shall show that \u2206DEF ~ \u2206ABC.
\nClearly, ED || AF and DE || EA.
\n\u2234 \u00a0\u00a0AFDE is a parallelogram.
\n\u21d2 \u00a0 \u2220EDF = \u2220A
\n[\u2235\u00a0 Opposite angles of a parallelogram are equal]
\nSimilarly, BDEF is a parallelogram.
\n\u2234 \u00a0\u2220DEF = \u2220B
\n[\u2235\u00a0 Opposite angles of a parallelogram are equal]
\nThus, in triangles DEF and ABC, we have
\n\u2220EDF = \u2220A and \u2220DEF = \u2220B
\nSo, by AA-criterion of similarity, we have
\n\u2206DEF ~ \u2206ABC.
\nThus, each one of the triangles AFE, FBD, EDC and DEF is similar to \u2206ABC.<\/p>\n
\nSol.<\/strong>\u00a0 \u00a0 \u00a0In \u2206ABC, we have
\n
\nDE || BC \u00a0 \u00a0\u00a0\\(\\Rightarrow \\frac{AB}{AD}=\\frac{AC}{AE}\\)
\nThus, in triangles ABC and ADE, we have
\n\\(\\frac{AB}{AD}=\\frac{AC}{AE}\\) \u00a0 \u00a0 \u00a0and, \u2220A = \u2220A
\nTherefore, by SAS-criterion of similarity, we have
\n\u2206ABC ~ \u2206ADE
\n\\(\\Rightarrow \\frac{AD}{AD}=\\frac{BC}{DE}\\) \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 ….(i)
\nIt is given that
\n\\( \\frac{AD}{DB}=\\frac{2}{3} \u00a0\\)
\n\\( \\Rightarrow \\frac{DB}{AD}=\\frac{3}{2} \\)
\n\\( \\Rightarrow \\frac{DB}{AD}+1=\\frac{3}{2}+1 \\)
\n\\( \\Rightarrow \\frac{DB+AD}{AD}=\\frac{5}{2} \\)
\n\\( \\Rightarrow \\frac{AB}{DE}=\\frac{5}{2} \u00a0\\)\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 ….(ii)
\nFrom (i) and (ii), we get
\n\\( \\frac{BC}{DE}=\\frac{5}{2} \\)<\/p>\n","protected":false},"excerpt":{"rendered":"