Class 10: Similarity – Exercise 16(a) – ICSE / ISC / CBSE Mathematics Portal for K12 Students

Question 1: State True or False:

(i) Two similar polygons are necessarily congruent – False

(ii) Two congruent polygons are necessarily similar – True

(iii) All equi-angular triangles are similar – True

(iv) All isosceles triangles are similar – False

(v) Two isosceles right angles triangles are similar – True

(vi) Two isosceles triangles are similar, if an angle of one is congruent to the corresponding angle of the other – True

(vii) The diagonals of the trapezium, divide each into proportional segments – True

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Question 2: In $\triangle ABC, DE \parallel BC$ , where $D and E$ are points on $AB and AC$ respectively. Prove that $\triangle ADE \sim \triangle ABC$ . Also find the length of $DE, \ if \ AD = 12 \ cm, BD = 24 \ cm \ and \ BC = 8 \ cm$ .

Answer:

In $\triangle ABC, DE \parallel BC$

Consider $\triangle ABC \ and\ \triangle ADE$

$\angle ABC = \angle ADE$ (alternate angles)

$\angle ACB =\angle AED$ (alternate angles)

Therefore $\triangle ADE \sim \triangle ABC$ (AAA postulate)

Hence $\frac{AD}{AB}$ $=$ $\frac{DE}{BC}$ $=$ $\frac{AE}{AC}$

$\frac{12}{12+24}$ $=$ $\frac{DE}{8}$

$\Rightarrow DE =$ $\frac{12}{36}$ $\times 8 =$ $\frac{8}{3}$

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Question 3: Given $\angle GHE = \angle DFE= 90^o$ , $DH= 8, DF=12,$ $DG=3x-1$ and $DE=4x+2$ . Find the length of the segments $DG \ and \ DE$

Answer:

Consider $\triangle EFD and \triangle GHD$

$\angle GHE = \angle DFE= 90^o$ (Given)

$\angle EDG = \angle HDG \ (\angle D \ is \ common)$

Therefore $\triangle EFD \sim \triangle GHD$ (AAA postulate)

$\frac{DH}{DF}$ $=$ $\frac{DG}{DE}$

$\frac{8}{12}$ $=$ $\frac{3x-1}{4x+2}$

$32x+16 = 36x-12$

$4x=28 \Rightarrow x = 7$

Substituting $DG = 3 \times 7 -1 = 20$

$DE = 4 \times 7 +2 = 30$

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Question 4: $D$ is a point of side $BC$ of $\triangle ABC$ such that $\angle ADC = \angle BAC$ . Prove that $CA^2=CB \times CD$ .

Answer: s12

Consider $\triangle ABC \ and \ \triangle ADC$

$\angle BAC = \angle ADC$ (Given)

$\angle ACB = \angle ACD \ (\angle C \ is \ common)$

Therefore $\triangle ABC \ and \ \triangle ADC$ (AAA postulate)

Therefore $\frac{CA}{CB}$ $=$ $\frac{CD}{CA}$

$\Rightarrow CA^2=CB \times CD$

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Question 5: sm10 In the given figure, $\triangle ABC$ and $\triangle AMP$ are right angled at $B$ and $M$ respectively. Given $AC=10 \ cm, AP = 15 \ cm$ and $PM= 12 \ cm$ . (i) Prove $\triangle ABC \sim \triangle AMP$ (ii) Find $AB$ and $AC$ . [2012]

Answer:

In $\triangle ABC \ and \ \triangle AMP$

$\angle BAC = \angle PAM$ (common angle)

$\angle ABC =\angle PMA$

Therefore $\triangle ABC \sim \triangle AMP$ (AAA postulate)

Since $\triangle ABC \sim \triangle AMP$

$\frac{AP}{AM}$ $=$ $\frac{BC}{PM}$ $=$ $\frac{AC}{AP}$

Given AC=10 cm, AP = 15 cm and PM= 12 cm

$\Rightarrow AB =$ $\frac{10}{15}$ $\times 11 = 7.33 \ cm$

$\Rightarrow BC =$ $\frac{10}{15}$ $\times 12 = 8 \ cm$

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Question 6: $E \ and \ F$ are points in sides $DC \ and \ AB$ respectively of parallelogram $ABCD$ . If diagonal $AC$ and segment $EF$ intersect at $G$ ; prove that: $AG \times EG = FG \times CG$ .

Answer: s10

In $\triangle AGF \ and \ \triangle EGC$

$\angle AGF = \angle EGC$ (opposite angles)

$\angle ECG =\angle FAG$ (alternate angles)

Therefore $\triangle AGF \sim \triangle ECG$ (AAA postulate)

Therefore $\frac{EG}{FG}$ $=$ $\frac{CG}{AG}$

$\Rightarrow AG \times EG = FG \times CG$

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Question 7: Given $RS \ and \ PT$ are altitudes of $\triangle PQR$ . Prove that (i) $\triangle PQT \sim \triangle QRS$ (ii) $PQ \times QS = RQ \times QT$

Answer:

(i) Consider $\triangle PQT \ and \ \triangle QRS$

$\angle QTP = \angle QSR= 90^o$ (Given, altitudes)

$\angle PQT = \angle SQR \ (\angle Q \ is \ common)$

Therefore $\triangle PQT \sim \triangle QRS$ (AAA postulate)

(ii) $\frac{PQ}{RQ}$ $=$ $\frac{QT}{QS}$

$PQ \times QS = RQ \times QT$

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Question 8: Given $ABCD$ is a rhombus, $DPR \ and \ CBR$ are straight lines. Prove that $DP \times CR = DC \times PR$ .

Answer:

In $\triangle DPA \ and \ \triangle RPC$

$\angle DPA = \angle RPC$ (vertically opposite angles)

$\angle PAD =\angle PCR$ (alternate angles)

Therefore $\triangle DPA \sim \triangle RPC$ (AAA postulate)

Therefore $\frac{DB}{PR}$ $=$ $\frac{AD}{CR}$

$\frac{DP}{PR}$ $=$ $\frac{DC}{CR}$ ( $AD = DC \ as \ ABCD$ is a Rhombus)

$\Rightarrow DP \times CR = DC \times PR$

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Question 9: Given $FB=FD, AE \perp FD \ and \ FC \perp AD$ . Prove $\frac{GB}{AD}$ $=$ $\frac{BC}{ED}$

Answer:

$FB=FD, AE \perp FD$ and $FC \perp AD$ .

Therefore $\angle FDB=\angle FBD$

Consider $\triangle AED \ and \ \triangle FCB$

$\angle AED = \angle FCB= 90^o$ (Given)

$\angle ADE = \angle FBC$

Therefore $\triangle AED \sim \triangle FBC$ (AAA postulate)

$\frac{AD}{FB}$ $=$ $\frac{ED}{BC}$

$\frac{FB}{AD}$ $=$ $\frac{BC}{ED}$

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Question 10: In $\triangle ABC, \angle B = 2 \angle C$ and the bisector of $\angle B$ meets $CA$ at point $D$ . Prove that (i) $\triangle ABC \sim \triangle ABD$ (ii) $DC:AD = BC:AB$

Answer:

Given $\triangle ABC, \angle B = 2 \angle C$

$\angle ABD = \angle DBC$

Therefore $\angle ABD = \angle DBC = \angle ACB$

Consider $\triangle ABC \ and \ \triangle ABD$

$\angle BAC = \angle DAB$ (Given)

$\angle ACB = \angle ABD$

Therefore $\triangle ABC \sim \triangle ABD$ (AAA postulate)

$\frac{BC}{BD}$ $\frac{AB}{AD}$

$\frac{BC}{AB}$ $=$ $\frac{BD}{AD}$

$\frac{BC}{AB}$ $=$ $\frac{DC}{AD}$ ( $\angle DBC = \angle DCB \Rightarrow DC = BD$ )

$DC:AD = BC:AB$

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Question 11: In $\triangle PQR, \angle Q=90^o$ and $QM \perp PR$ . Prove that:

(i) $PQ^2 = PM \times PR$

(ii) $QR^2 = PR \times MR$

(iii) $PQ^2 + QR^2 = PR^2$

Answer:

(i) Given $\triangle PQR, \angle Q=90^o$ and $QM \perp PR$

Consider $\triangle PQM \ and\ \triangle PRQ$

$\angle PMQ = \angle PQR = 90^o$ (given)

$\angle QPM =\angle RPQ$ (common angle)

Therefore $\triangle PQM \sim \triangle PRQ$ (AAA postulate)

Hence $\frac{PQ}{PR}$ $=$ $\frac{PM}{PQ}$

Therefore $PQ^2 = PM \times PR$

(ii) Consider $\triangle QMR \ and\ \triangle PQR$

$\angle QMR = \angle PQR = 90^o$ (given)

$\angle QRM =\angle QRP$ (common angle)

Therefore $\triangle QMR \sim \triangle PQR$ (AAA postulate)

Hence $\frac{QR}{PR}$ $=$ $\frac{MR}{QR}$

Therefore $QR^2 = PR \times MR$

(iii) Adding (i) and (ii)

$PQ^2 + QR^2 = PM \times PR + PR \times MR$

$\Rightarrow PQ^2 + QR^2 = PR \times (PM + MR)$

$\Rightarrow PQ^2 + QR^2 = PR \times PR = PR^2$

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Question 12: In $\triangle ABC, \angle C= 90^o, CD \perp AB$ . Prove that $CD^2 = AD \times DB$ .

Answer: s16

$\triangle ABC, \angle C= 90^o, CD \perp AB$ .

Consider $\triangle PQM \ and\ \triangle PRQ$

$\angle PMQ = \angle PQR = 90^o$ (given)

$\angle QPM =\angle RPQ$ (common angle)

Therefore $\triangle PQM \sim \triangle PRQ$ (AAA postulate)

Hence $\frac{PQ}{PR}$ $=$ $\frac{PM}{PQ}$

Therefore $PQ^2 = PM \times PR$

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Question 13: In $\triangle ABC, \angle B= 90^o, BD \perp AC$ .

(i) If $CD=10 \ cm \ and \ BD = 8 \ cm, \ find \ AD$

(ii) If $AC=18 \ cm \ and \ AD = 6 \ cm, \ find \ BD$

(iii) If $AC=9 \ cm \ and \ AB = 7 \ cm, \ find \ AD$

Answer: s16

$\triangle ABC, \angle B= 90^o, BD \perp AC$

Consider $\triangle CDB \ and\ \triangle BCD$

$\angle ADB = \angle BDC = 90^o$ (given)

$\angle ABD =\angle BCD$

Therefore $\triangle CDB \sim \triangle BDA$ (AAA postulate)

(i) $\frac{CD}{BD}$ $=$ $\frac{BD}{AD}$

Therefore $AD =$ $\frac{BD^2}{CD}$ $=$ $\frac{8^2}{10}$ $= 6.4\ cm$

(ii) $\frac{BD}{DA}$ $=$ $\frac{CD}{BD}$

Therefore $BD = \sqrt{CD \times DA} = \sqrt{72}= 8.49 \ cm$

(iii) $\frac{AD}{AB}$ $=$ $\frac{AB}{AC}$

Therefore $AD =$ $\frac{AB \times AB}{AC}$ $=$ $\frac{7 \times 7}{9}$ $= 5.44 \ cm$

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Question 14: sm11 In the figure, $PQRS$ is a parallelogram with $PQ=16 \ cm$ and $QR=10 \ cm$ . $L$ is a point on $PR$ such that $RL:LP=2:3$ . $QL$ produced meets $RS$ at $M$ and $PS$ produced at $N$ . Find the lengths of $PN$ and $RM$ . [1997]

Answer:

In $\triangle RLQ$ and $\triangle PLN$

$\angle RLQ = \angle PLN$ (vertically opposite angles)

$\angle LRQ = \angle LPN$ (alternate angles)

Therefore $\triangle RLQ \sim \triangle PLN$ (AAA postulate)

Therefore $\frac{RL}{LP}$ $=$ $\frac{RQ}{PN}$

$\Rightarrow PN =$ $\frac{10}{2}$ $\times 3 = 15 \ cm$

In $\triangle RLM \ and \ \triangle PLQ$

$\angle RLM = \angle PLQ$ (vertically opposite angles)

$\angle LRM = \angle LPQ$ (alternate angles)

Therefore $\triangle RLM \sim \triangle PLQ$ (AAA postulate)

Therefore $\frac{RM}{PQ}$ $=$ $\frac{RL}{LP}$

$\Rightarrow RM =$ $\frac{2}{3}$ $\times 16 = 10.667 \ cm$

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Question 15: In quadrilateral $ABCD$ , Diagonal $AC \ and \ BD$ intersect at point $E$ such that: $AE:EC=BE:ED$ . Show that $ABCD$ is a parallelogram.

Answer:

Given $\frac{AE}{EC}$ $=$ $\frac{BE}{ED}$ … … … … (i)

In $\triangle ADB$ , because $EF \parallel AB$ , by proportionality theorem

$\frac{DF}{FA}$ $=$ $\frac{ED}{BE}$ … … … … (i)

Therefore from (i) and (ii)

$\frac{DF}{FA}$ $=$ $\frac{EC}{AE}$

Similarly in $\triangle DCA$

$\frac{DF}{FA}$ $=$ $\frac{CE}{AE}$ $\Rightarrow FE \parallel DC$

Also it is given that $FE \parallel AB$

Therefore $DC \parallel AB$ . Therefore $ABCD$ is a parallelogram.

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Question 16: Given $AB \parallel DE$ and $BC \parallel EF$ . Prove that:

(i) $\frac{AD}{DG}$ $=$ $\frac{CF}{FG}$

(ii) $\triangle DFG \sim \triangle ACG$

Answer:

(i) Given $AB \parallel DE$ and $BC \parallel EF$

In $\triangle AGB,$ $\frac{AD}{DG}$ $=$ $\frac{EB}{GE}$ … … … … (i)

In $\triangle BGC,$ $\frac{EB}{GE}$ $=$ $\frac{CF}{FG}$ … … … … (ii)

From (i) and (ii) you get $\frac{AD}{DG}$ $=$ $\frac{CF}{FG}$

(ii) Consider $\triangle DFG \ and\ \triangle ACG$

We have $\frac{AD}{DG}$ $=$ $\frac{CF}{FG}$

and $\angle DGF = \angle AGC$ (common angle)

Therefore $\triangle DFG \sim \triangle ACG$

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Question 17: In $\triangle ABC, AD \perp BC$ and $AD^2 = BD \times DC$ . Show that $\angle BAC = 90^o$ .

Answer:

Given $AD^2 = BD \times DC$

$\Rightarrow \frac{AD}{DC}$ $=$ $\frac{BD}{AD}$

Also given $\angle ADB = \angle ADC = 90^o$

Therefore $\triangle DBA \sim \triangle DAC$ (SAS postulate)

Therefore $\angle C = \angle BAD$

$\angle B = \angle DAC$

Adding $\angle C + \angle B = \angle BAD + \angle DAC$

$\Rightarrow \angle A =\angle C + \angle B$ … … … (i)

We know $\angle A + \angle B + \angle C = 180^o$ … … … … (ii)

From (i) and (ii) $2 \angle A = 180^o$

$\Rightarrow \angle A = 90^o$

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Question 18: In the given figure $AB \parallel EF \parallel DC$ ; $AB=67.5 \ cm, DC = 40.5 \ cm$ and $AE = 52.5 \ cm$ .

(i) Name the three pairs of similar triangles

(ii) Find the lengths of $EC \ and \ EF$

Answer:

Given $AB \parallel EF \parallel DC$ , The three pairs of similar triangles are

$\triangle ABC \sim \triangle CEF$

$\triangle BCD \sim \triangle BEF$

$\triangle ABE \sim \triangle CDE$

Since $\triangle ABE \sim \triangle CDE$

$\frac{AB}{CD}$ $=$ $\frac{AE}{CE}$ $\Rightarrow CE =$ $\frac{52.5 \times 40.5}{67.5}$ $= 31.5 \ cm$

Since $\triangle ABC \sim \triangle CEF$

$\frac{CE}{CA}$ $=$ $\frac{EF}{AB}$ $\Rightarrow EF =$ $\frac{67.5 \times 31.5}{52.5+31.5}$ $=$ $25.3125 \ cm$

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Question 19: In the given figure $QR \parallel AB$ , and $DR \parallel QB$ . Prove that $PQ^2 = PD \times PA$ .

Answer:

Given $QR \parallel AB$ , and $DR \parallel QB$

Using basic proportionality theorem

In $\triangle PQR \ and \ \triangle PAB$

$\frac{PQ}{PA}$ $=$ $\frac{PR}{PB}$ … … … … (i)

In $\triangle PDR \ and \ \triangle PQB$

$\frac{PD}{PQ}$ $=$ $\frac{PR}{PB}$ … … … … (ii)

From (i) and (ii) we get

$\frac{PQ}{PA}$ $=$ $\frac{PD}{PQ}$

$\Rightarrow PQ^2 = PD \times PA$

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Question 20: Through the mid point $M$ of the side $CD$ of a parallelogram $ABCD$ , the line $BM$ is drawn intersecting diagonal $AC \ in \ L$ and $AD$ produced in $E$ . Prove that $EL=2BL$ .

Answer:

Consider $\triangle DEM \ and \ \triangle CBM$

$\angle AEB = \angle EBC$ (alternate angles)

$\angle EMD = \angle CMB$ (opposite angles)

$DM = MC$ (as M is the midpoint of CD)

Therefore $\triangle DEM \cong \triangle CBM$

Therefore $DE = BC$ (corresponding angles)

$AD = BC$ (opposite sides of a parallelogram)

Therefore $AE = AD + DE = 2BC$

Now consider $\triangle ELA \ and \ \triangle LBC$

$\angle AEB = \angle EBC$

$\angle ELA = \angle EBC$

Therefore $\triangle ELA \sim \triangle LBC$

Hence $\frac{EL}{BL}$ $=$ $\frac{EA}{BC}$

$\frac{EL}{BL}$ $=$ $\frac{2BC}{BC}$ $= 2$

Hence $EL = 2 BL$

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Question 21: sm12 In the figure given below, $P$ is a point on $AB$ such that $AP:PB=4:3$ . $PQ \parallel AC$ .

(i) Calculate the ratio $PQ:AC$ , giving reasons for your answer.

(ii) In $\triangle ARC, \angle ARC = 90^o$ and in $\triangle PQS, \angle PSQ=90^o$ . Given $QS=6 \ cm$ , calculate length of $AR$ . [1999]

Answer:

(i) Given $AP:PB=4:3$

Also $PQ \parallel AC$ , applying basic proportionality theorem

$\frac{AP}{PB}$ $=$ $\frac{CQ}{QB}$

$\Rightarrow$ $\frac{CQ}{QB}$ $=$ $\frac{4}{3}$

$\Rightarrow$ $\frac{BQ}{BC}$ $=$ $\frac{3}{7}$

$\angle QBP = \angle ACB$ (corresponding angles)

$\angle QPB = \angle CAB$ (corresponding angles)

Therefore $\triangle PBQ \sim \triangle ABC$ (AAA postulate)

$\frac{PQ}{ AC}$ $=$ $\frac{BQ}{ BC}$ $=$ $\frac{3}{ 7}$

(ii) Given $\angle ARC = \angle QSP = 90^o$

$\angle ACR = \angle SPQ$ (alternate angles)

Therefore $\triangle ARC \sim \triangle QSP$ (AAA postulate)

$\frac{AR}{ SQ}$ $=$ $\frac{AC}{ PQ}$ $=$ $\frac{7}{ 3}$

$\Rightarrow AR =$ $\frac{7 \times 6}{3}$ $= 14 \ cm$

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Question 22: In the right angled $\triangle QPR, PM$ is the altitude. Given that $QR= 8 cm$ and $MQ = 3.5 cm$ , calculate the value of $PR$ . [2000]