Question

NCERT Solutions for Class 9 Maths Chapter 7 Triangles are provided here which is one of the most important study materials for the students preparing for the CBSE board examination. Our NCERT Solutions are based on the latest syllabus of the CBSE. Prepared and designed by subject matter experts these solutions are on to the point and explained in a point-wise method to make it easier for the students to learn, study, and do last-minute revision. We have also covered all the shortcuts, equations, rules, theorems, and axioms.

In the NCERT Solution Class 9 provided by us, we have explained all the topics in detail which will surely help students clear all the required concepts. Important topics discussed here are:

• Congruent Circles
• Congruent Triangles
• Triangles' Postulates of Congruency
• Sufficient Condition for Two Triangles to Be Congruent
• Activity
• SAS Congruency Condition
• Theorem 9
• Application of SAS Congruency
• Theorem 10
• ASA Congruence Condition:
• Theorem 11
• Theorem 12
• AAS Congruence Condition
• SSS Congruence Condition
• Theorem of RHS Congruence
• Inequality of Angles
• Inequality in a Triangle

Our NCERT Solutions Class 9 Maths is the best resource to practice and learn about all the important concepts. Regular practice of our solutions will help students develop problem-solving skills related to the subject matter. We have solutions for all kinds of queries including exercise and in-text questions.

Answer 1

NCERT Solutions for Class 9 Maths Chapter 7 Triangles

1. In quadrilateral ACBD, AC = AD and AB bisects ∠A (see figure). Show that ∆ABC ≅ ∆ABD. What can you say about BC and BD?

Solution:

In quadrilateral ACBD, we have AC = AD and AB being the bisector of ∠A.

Now, In ∆ABC and ∆ABD,

AC = AD (Given)

∠CAB = ∠DAB ( AB bisects ∠CAB)

and AB = AB (Common)

∴ ∆ ABC ≅ ∆ABD (By SAS congruence axiom)

∴ BC = BD (By CPCT)

2. ABCD is a quadrilateral in which AD = BC and ∠ DAB = ∠ CBA (see figure). Prove that

(i) ∆ABD ≅ ∆BAC

(ii) BD = AC

(iii) ∠ABD = ∠BAC

Solution:

In quadrilateral ACBD, we have AD = BC and ∠DAB = ∠CBA

(i) In ∆ ABC and ∆ BAC,

AD = BC (Given)

∠DAB = ∠CBA (Given)

AB = AB (Common)

∴ ∆ ABD ≅ ∆BAC (By SAS congruence)

(ii) Since ∆ABD ≅ ∆BAC

⇒ BD = AC [By C.P.C.T.]

(iii) Since ∆ABD ≅ ∆BAC

⇒ ∠ABD = ∠BAC [By C.P.C.T.]

3. AD and BC are equal perpendiculars to a line segment AB (see figure). Show that CD bisects AB.

Solution:

In ∆BOC and ∆AOD, we have

∠BOC = ∠AOD

BC = AD [Given]

∠BOC = ∠AOD   [Vertically opposite angles]

∴ ∆OBC ≅ ∆OAD   [By AAS congruency]

⇒ OB = OA   [By C.P.C.T.]

i.e., O is the mid-point of AB.

Thus, CD bisects AB.

4. l and m are two parallel lines intersected by another pair of parallel lines p and q (see figure). Show that ∆ABC = ∆CDA.

Solution:

∵ p || q and AC is a transversal,

∴ ∠BAC = ∠DCA    …(1) [Alternate interior angles]

Also l || m and AC is a transversal,

∴ ∠BCA = ∠DAC    …(2)

[Alternate interior angles]

Now, in ∆ABC and ∆CDA, we have

∠BAC = ∠DCA    [From (1)]

CA = AC   [Common]

∠BCA = ∠DAC    [From (2)]

∴ ∆ABC ≅ ∆CDA   [By ASA congruency]

5. Line l is the bisector of an ∠ A and ∠ B is any point on l. BP and BQ are perpendiculars from B to the arms of LA (see figure). Show that

(i) ∆APB ≅ ∆AQB

(ii) BP = BQ or B is equidistant from the arms of ∠A.

Solution:

We have, l is the bisector of ∠QAP.

∴ ∠QAB = ∠PAB

∠Q = ∠P   [Each 90°]

∠ABQ = ∠ABP

[By angle sum property of A]

Now, in ∆APB and ∆AQB, we have

∠ABP = ∠ABQ   [Proved above]

AB = BA   [Common]

∠PAB = ∠QAB   [Given]

∴ ∆APB ≅ ∆AQB   [By ASA congruency]

Since ∆APB ≅ ∆AQB

⇒ BP = BQ   [By C.P.C.T.]

i.e., [Perpendicular distance of B from AP]

= [Perpendicular distance of B from AQ]

Thus, the point B is equidistant from the arms of ∠A.

6. In figure, AC = AE, AB = AD and ∠BAD = ∠EAC. Show that BC = DE.

Solution:

We have, ∠BAD = ∠EAC

Adding ∠DAC on both sides, we have

∠BAD + ∠DAC = ∠EAC + ∠DAC

⇒ ∠BAC = ∠DAE

Now, in ∆ABC and ∆ADE. we have

∠BAC = ∠DAE  [Proved above]

AB = AD  [Given]

AC = AE  [Given]

∴ ∆ABC ≅ ∆ADE  [By SAS congruency]

⇒ BC = DE  [By C.P.C.T.]

7. AS is a line segment and P is its mid-point. D and E are points on the same side of AB such that ∠BAD = ∠ABE and ∠EPA = ∠DPB. (see figure). Show that

(i) ∆DAP ≅ ∆EBP

(ii) AD = BE

Solution:

We have, P is the mid-point of AB.

∴ AP = BP

∠EPA = ∠DPB [Given]

Adding ∠EPD on both sides, we get

∠EPA + ∠EPD = ∠DPB + ∠EPD

⇒ ∠APD = ∠BPE

(i) Now, in ∆DAP and ∆EBP, we have

∠PAD = ∠PBE  [ ∵∠BAD = ∠ABE]

AP = BP [Proved above]

∠DPA = ∠EPB  [Proved above]

∴ ∆DAP ≅ ∆EBP  [By ASA congruency]

(ii) Since, ∆DAP ≅ ∆EBP

⇒ AD = BE  [By C.P.C.T.]

Answer 2

8. In right triangle ABC, right angled at C, M is the mid-point of hypotenuse AB. C is joined to M and produced to a point D such that DM = CM. Point D is joined to point B (see figure). Show that

(i) ∆AMC ≅ ∆BMD

(ii) ∠DBC is a right angle

(iii) ∆DBC ≅ ∆ACB

(iv) CM = \(\frac12\) AB

Solution:

Since M is the mid – point of AB.

∴ BM = AM

(i) In ∆AMC and ∆BMD, we have

CM = DM [Given]

∠AMC = ∠BMD [Vertically opposite angles]

AM = BM [Proved above]

∴ ∆AMC ≅ ∆BMD [By SAS congruency]

(ii) Since ∆AMC ≅ ∆BMD

⇒ ∠MAC = ∠MBD [By C.P.C.T.]

But they form a pair of alternate interior angles.

∴ AC || DB

Now, BC is a transversal which intersects parallel lines AC and DB,

∴ ∠BCA + ∠DBC = 180° [Co-interior angles]

But ∠BCA = 90° [∆ABC is right angled at C]

∴ 90° + ∠DBC = 180°

⇒ ∠DBC = 90°

(iii) Again, ∆AMC ≅ ∆BMD [Proved above]

∴ AC = BD [By C.P.C.T.]

Now, in ∆DBC and ∆ACB, we have

BD = CA   [Proved above]

∠DBC = ∠ACB  [Each 90°]

BC = CB  [Common]

∴ ∆DBC ≅ ∆ACB  [By SAS congruency]

(iv) As ∆DBC ≅ ∆ACB

DC = AB [By C.P.C.T.]

But DM = CM [Given]

∴ CM = \(\frac12\)DC = \(\frac12\)AB

⇒ CM = \(\frac12\)AB

9. In an isosceles triangle ABC, with AB = AC, the bisectors of ∠B and ∠C intersect each other at 0. Join A to 0. Show that

(i) OB = OC

(ii) AO bisects ∠A

Solution:

(i) in ∆ABC, we have

AB = AC [Given]

∴ ∠ABC = ∠ACB [Angles opposite to equal sides of a A are equal]

⇒ \(\frac12\)∠ABC = \(\frac12\)∠ACB

or ∠OBC = ∠OCB

⇒ OC = OB [Sides opposite to equal angles of a ∆ are equal]

(ii) In ∆ABO and ∆ACO, we have

AB = AC [Given]

∠OBA = ∠OCA   [∵ \(\frac12\)∠B = \(\frac12\)∠C]

OB = OC [Proved above]

∆ABO ≅ ∆ACO [By SAS congruency]

⇒ ∠OAB = ∠OAC [By C.P.C.T.]

⇒ AO bisects ∠A.

10. In ∆ABC, AD is the perpendicular bisector of BC (see figure). Show that ∆ ABC is an isosceles triangle in which AB = AC.

Solution:

Since AD is bisector of BC.

∴ BD = CD

Now, in ∆ABD and ∆ACD, we have

AD = DA [Common]

∠ADB = ∠ADC [Each 90°]

BD = CD [Proved above]

∴ ∆ABD ≅ ∆ACD [By SAS congruency]

⇒ AB = AC [By C.P.C.T.]

Thus, ∆ABC is an isosceles triangle.

11. ABC is an isosceles triangle in which altitudes BE and CF are drawn to equal sides AC and AB respectively (see figure). Show that these altitudes are equal.

Solution:

∆ABC is an isosceles triangle.

∴ AB = AC

⇒ ∠ACB = ∠ABC [Angles opposite to equal sides of a A are equal]

⇒ ∠BCE = ∠CBF

Now, in ∆BEC and ∆CFB

∠BCE = ∠CBF [Proved above]

∠BEC = ∠CFB [Each 90°]

BC = CB [Common]

∴ ∆BEC ≅ ∆CFB [By AAS congruency]

So, BE = CF [By C.P.C.T.]

12. ABC is a triangle in which altitudes BE and CF to sides AC and AB are equal (see figure).

Show that

(i) ∆ABE ≅ ∆ACF

(ii) AB = AC i.e., ABC is an isosceles triangle.

Solution:

(i) In ∆ABE and ∆ACE, we have

∠AEB = ∠AFC

[Each 90° as BE ⊥ AC and CF ⊥ AB]

∠A = ∠A [Common]

BE = CF [Given]

∴ ∆ABE ≅ ∆ACF [By AAS congruency]

(ii) Since, ∆ABE ≅ ∆ACF

∴ AB = AC [By C.P.C.T.]

⇒ ABC is an isosceles triangle.

13. ABC and DBC are isosceles triangles on the same base BC (see figure). Show that ∠ABD = ∠ACD.

Solution:

In ∆ABC, we have

AB = AC [ABC is an isosceles triangle]

∴ ∠ABC = ∠ACB    …(1)

[Angles opposite to equal sides of a ∆ are equal]

Again, in ∆BDC, we have

BD = CD [BDC is an isosceles triangle]

∴ ∠CBD = ∠BCD    …(2)

[Angles opposite to equal sides of a A are equal]

Adding (1) and (2), we have

∠ABC + ∠CBD = ∠ACB + ∠BCD

⇒ ∠ABD = ∠ACD.

14. ∆ABC is an isosceles triangle in which AB = AC. Side BA is produced to D such that AD = AB (see figure). Show that ∠BCD is a right angle.

Solution:

AB = AC [Given]   …(1)

AB = AD [Given]   …(2)

From (1) and (2), we have

AC = AD

Now, in ∆ABC, we have

∠ABC + ∠ACB + ∠BAC = 180° [Angle sum property of a A]

⇒ 2∠ACB + ∠BAC = 180° …(3)

[∠ABC = ∠ACB (Angles opposite to equal sides of a A are equal)]

Similarly, in ∆ACD,

∠ADC + ∠ACD + ∠CAD = 180°

⇒ 2∠ACD + ∠CAD = 180° …(4)

[∠ADC = ∠ACD (Angles opposite to equal sides of a A are equal)]

Adding (3) and (4), we have

2∠ACB + ∠BAC + 2 ∠ACD + ∠CAD = 180° +180°

⇒ 2[∠ACB + ∠ACD] + [∠BAC + ∠CAD] = 360°

⇒ 2∠BCD +180° = 360°  [∠BAC and ∠CAD form a linear pair]

⇒ 2∠BCD = 360° – 180° = 180°

⇒ ∠BCD = \(\frac{180°}2\) = 90°

Thus, ∠BCD = 90°

Answer 3

15. ABC is a right angled triangle in which ∠A = 90° and AB = AC, find ∠B and ∠C.

Solution:

In ∆ABC, we have AB = AC [Given]

∴ Their opposite angles are equal.

⇒ ∠ACB = ∠ABC

Now, ∠A + ∠B + ∠C = 180°   [Angle sum property of a ∆]

⇒ 90° + ∠B + ∠C = 180°   [∠A = 90°(Given)]

⇒ ∠B + ∠C= 180°- 90° = 90°

But ∠B = ∠C

∠B = ∠C = \(\frac{90°}2\) = 45°

Thus, ∠B = 45° and ∠C = 45°

16. Show that the angles of an equilateral triangle are 60° each.

Solution:

In ∆ABC, we have

AB = BC = CA

[ABC is an equilateral triangle]

AB = BC

⇒ ∠A = ∠C  …(1) [Angles opposite to equal sides of a A are equal]

Similarly, AC = BC

⇒ ∠A = ∠B   …(2)

From (1) and (2), we have

∠A = ∠B = ∠C = x (say)

Since, ∠A + ∠B + ∠C = 180° [Angle sum property of a A]

∴ x + x + x = 180o

⇒ 3x = 180°

⇒ x = 60°

∴ ∠A = ∠B = ∠C = 60°

Thus, the angles of an equilateral triangle are 60° each.

17. ∆ABC and ∆DBC are two isosceles triangles on the same base BC and vertices A and D are on the same side of BC (see figure). If AD is extended to intersect BC at P, show that

(i) ∆ABD ≅ ∆ACD

(ii) ∆ABP ≅ ∆ACP

(iii) AP bisects ∠A as well as ∠D

(iv) AP is the perpendicular bisector of BC.

Solution:

(i) In ∆ABD and ∆ACD, we have

AB = AC [Given]

AD = DA [Common]

BD = CD [Given]

∴ ∆ABD ≅ ∆ACD [By SSS congruency]

∠BAD = ∠CAD [By C.P.C.T.] …(1)

(ii) In ∆ABP and ∆ACP, we have

AB = AC [Given]

∠BAP = ∠CAP [From (1)]

∴ AP = PA [Common]

∴ ∆ABP ≅ ∆ACP [By SAS congruency]

(iii) Since, ∆ABP ≅ ∆ACP

⇒ ∠BAP = ∠CAP [By C.P.C.T.]

∴ AP is the bisector of ∠A.

Again, in ∆BDP and ∆CDP,

we have BD = CD [Given]

DP = PD [Common]

BP = CP [ ∵ ∆ABP ≅ ∆ACP]

⇒ A BDP = ACDP [By SSS congruency]

∴ ∠BDP = ∠CDP [By C.P.C.T.]

⇒ DP (or AP) is the bisector of ∠BDC

∴ AP is the bisector of ∠A as well as ∠D.

(iv) As, ∆ABP ≅ ∆ACP

⇒ ∠APS = ∠APC, BP = CP [By C.P.C.T.]

But ∠APB + ∠APC = 180° [Linear Pair]

∴ ∠APB = ∠APC = 90°

⇒ AP ⊥ BC, also BP = CP

Hence, AP is the perpendicular bisector of BC.

18. AD is an altitude of an isosceles triangle ABC in which AB = AC. Show that

(i) AD bisects BC

(ii) AD bisects ∠A

Solution:

(i) In right ∆ABD and ∆ACD, we have

AB = AC   [Given]

∠ADB = ∠ADC [Each 90°]

AD = DA [Common]

∴ ∆ABD ≅ ∆ACD [By RHS congruency]

So, BD = CD [By C.P.C.T.]

⇒ D is the mid-point of BC or AD bisects BC.

(ii) Since, ∆ABD ≅ ∆ACD,

⇒ ∠BAD = ∠CAD [By C.P.C.T.]

So, AD bisects ∠A

19. Two sides AB and BC and median AM of one triangle ABC are respectively equal to sides PQ and OR and median PN of ∆PQR (see figure). Show that

(i) ∆ABC ≅ ∆PQR

(ii) ∆ABM ≅ ∆PQN

Solution:

In ∆ABC, AM is the median.

∴ BM = \(\frac12\)BC  ……(1)

In ∆PQR, PN is the median.

∴ QN = \(\frac12\)QR   …(2)

And BC = QR [Given]

⇒ 12BC = \(\frac12\)QR

⇒ BM = QN   …(3)  [From (1) and (2)]

(i) In ∆ABM and ∆PQN, we have

AB = PQ , [Given]

AM = PN [Given]

BM = QN [From (3)]

∴ ∆ABM ≅ ∆PQN [By SSS congruency]

(ii) Since ∆ABM ≅ ∆PQN

⇒ ∠B = ∠Q …(4) [By C.P.C.T.]

Now, in ∆ABC and ∆PQR, we have

∠B = ∠Q  [From (4)]

AB = PQ   [Given]

BC = QR   [Given]

∴ ∆ABC ≅ ∆PQR   [By SAS congruency]

20. BE and CF are two equal altitudes of a triangle ABC. Using RHS congruence rule, prove that the triangle ABC is isosceles.

Solution:

Since BE ⊥ AC [Given]

∴ BEC is a right triangle such that ∠BEC = 90°

Similarly, ∠CFB = 90°

Now, in right ∆BEC and ∆CFB, we have

BE = CF   [Given]

BC = CB   [Common hypotenuse]

∠BEC = ∠CFB   [Each 90°]

∴ ∆BEC ≅ ∆CFB   [By RHS congruency]

So, ∠BCE = ∠CBF   [By C.P.C.T.]

or ∠BCA = ∠CBA

Now, in ∆ABC, ∠BCA = ∠CBA

⇒ AB = AC  [Sides opposite to equal angles of a ∆ are equal]

∴ ABC is an isosceles triangle.

21. ABC is an isosceles triangle with AB = AC. Draw AP ⊥ BC to show that ∠B = ∠C.

Solution:

We have, AP ⊥ BC [Given]

∠APB = 90° and ∠APC = 90°

In ∆ABP and ∆ACP, we have

∠APB = ∠APC  [Each 90°]

AB = AC  [Given]

AP = AP  [Common]

∴ ∆ABP ≅ ∆ACP  [By RHS congruency]

So, ∠B = ∠C  [By C.P.C.T.]

Answer 4

22. Show that in a right angled triangle, the hypotenuse is the longest side.

Solution:

Let us consider ∆ABC such that ∠B = 90°

∴ ∠A + ∠B + ∠C = 180°

⇒ ∠A + 90°-+ ∠C = 180°

⇒ ∠A + ∠C = 90°

⇒∠A + ∠C = ∠B

∴ ∠B > ∠A and ∠B > ∠C

⇒ Side opposite to ∠B is longer than the side opposite to ∠A

i.e., AC > BC.

Similarly, AC > AB.

Therefore, we get AC is the longest side. But AC is the hypotenuse of the triangle. Thus, the hypotenuse is the longest side.

23. In figure, sides AB and AC of ∆ABC are extended to points P and Q respectively. Also, ∠PBC < ∠QCB. Show that AC > AB.

Solution:

∠ABC + ∠PBC = 180°  [Linear pair]

and ∠ACB + ∠QCB = 180°  [Linear pair]

But ∠PBC < ∠QCB [Given]

⇒ 180° – ∠PBC > 180° – ∠QCB

⇒ ∠ABC > ∠ACB

The side opposite to ∠ABC > the side opposite to ∠ACB

⇒ AC > AB.

24. In figure, ∠B < ∠A and ∠C < ∠D. Show that AD < BC.

Solution:

Since ∠A > ∠B [Given]

∴ OB > OA   …(1)

[Side opposite to greater angle is longer]

Similarly, OC > OD  …(2)

Adding (1) and (2), we have

OB + OC > OA + OD

⇒ BC > AD

25. AB and CD are respectively the smallest and longest sides of a quadrilateral ABCD (see figure). Show that ∠A > ∠C and ∠B > ∠D.

Solution:

Let us join AC.

Now, in ∆ABC, AB < BC [∵ AB is the smallest side of the quadrilateral ABCD]

⇒ BC > AB

⇒ ∠BAC > ∠BCA   …(1)

[Angle opposite to longer side of A is greater]

Again, in ∆ACD, CD > AD

[ CD is the longest side of the quadrilateral ABCD]

⇒ ∠CAD > ∠ACD  …(2)

[Angle opposite to longer side of ∆ is greater]

Adding (1) and (2), we get

∠BAC + ∠CAD > ∠BCA + ∠ACD

⇒ ∠A > ∠C

Similarly, by joining BD, we have ∠B > ∠D.

26. In figure, PR > PQ and PS bisect ∠QPR. Prove that ∠PSR > ∠PSQ.

Solution:

In ∆PQR, PS bisects ∠QPR [Given]

∴ ∠QPS = ∠RPS

and PR > PQ [Given]

⇒ ∠PQS > ∠PRS [Angle opposite to longer side of A is greater]

⇒ ∠PQS + ∠QPS > ∠PRS + ∠RPS  …(1)  [∵∠QPS = ∠RPS]

∵ Exterior ∠PSR = [∠PQS + ∠QPS]

and exterior ∠PSQ = [∠PRS + ∠RPS]

[An exterior angle is equal to the sum of interior opposite angles]

Now, from (1), we have

∠PSR = ∠PSQ.

27. Show that of all line segments drawn from a given point not on it, the perpendicular line segment is the shortest.

Solution:

Let us consider the ∆PMN such that ∠M = 90°

Since, ∠M + ∠N+ ∠P = 180°

[Sum of angles of a triangle is 180°]

∵ ∠M = 90° [PM ⊥ l]

So, ∠N + ∠P = ∠M

⇒ ∠N < ∠M

⇒ PM < PN   …(1)

Similarly, PM < PN₁   …(2)

and PM < PN₂  …(3)

From (1), (2) and (3), we have

PM is the smallest line segment drawn from P on the line l. Thus, the perpendicular line segment is the shortest line segment drawn on a line from a point not on it.

28. ABC is a triangle. Locate a point in the interior of ∆ ABC which is equidistant from all the vertices of ∆ ABC.

Solution:

Let us consider a ∆ABC.

Draw l, the perpendicular bisector of AB.

Draw m, the perpendicular bisector of BC.

Let the two perpendicular bisectors l and m meet at O.

O is the required point which is equidistant from A, B and C.

Note: If we draw a circle with centre O and radius OB or OC, then it will pass through A, B and C. The point O is called circumcentre of the triangle.

29. In a triangle locate a point in its interior which is equidistant from all the sides of the triangle.

Solution:

Let us consider a ∆ABC.

Draw m, the bisector of ∠C.

Let the two bisectors l and m meet at O.

Thus, O is the required point which is equidistant from the sides of ∆ABC.

Note: If we draw OM ⊥ BC and draw a circle with O as centre and OM as radius, then the circle will touch the sides of the triangle. Point O is called incentre of the triangle.

30. Complete the hexagonal and star shaped Rangolies [see Fig. (i) and (ii)] by filling them with as many equilateral triangles of side 1 cm as you can. Count the number of triangles in each case. Which has more triangles?

Solution:

It is an activity.

We require 150 equilateral triangles of side 1 cm in the Fig. (i) and 300 equilateral triangles in the Fig. (ii).

∴ The Fig. (ii) has more triangles.