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Quadrilaterals — NCERT Solutions

Madhya Pradesh Board · Class 9 · Mathematics

NCERT Solutions for Quadrilaterals, Madhya Pradesh Board Class 9 Mathematics: 13 textbook questions solved step by step.

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A labeled diagram of a parallelogram ABCD with diagonals AC and BD intersecting at point O, showing that AO=OC and BO=OD with tick marks.
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Exercise 8.1

1If the diagonals of a parallelogram are equal, then show that it is a rectangle.Show solution

Given: ABCD is a parallelogram in which diagonal AC = diagonal BD.

To prove: ABCD is a rectangle.

Proof:

In △ABC\triangle ABC and △DCB\triangle DCB:

  • AB=DCAB = DC (opposite sides of a parallelogram)
  • BC=BCBC = BC (common)
  • AC=DBAC = DB (given, diagonals are equal)

By SSS congruence rule:
△ABC≅△DCB\triangle ABC \cong \triangle DCB

Therefore, ∠ABC=∠DCB\angle ABC = \angle DCB (CPCT)

Since AB∥DCAB \parallel DC and BCBC is a transversal:
∠ABC+∠DCB=180∘(co-interior angles)\angle ABC + \angle DCB = 180^\circ \quad \text{(co-interior angles)}

⇒2∠ABC=180∘\Rightarrow 2\angle ABC = 180^\circ

⇒∠ABC=90∘\Rightarrow \angle ABC = 90^\circ

Since ABCD is a parallelogram with one angle =90∘= 90^\circ, ABCD is a rectangle. ■\blacksquare

2Show that the diagonals of a square are equal and bisect each other at right angles.Show solution

Given: ABCD is a square, i.e., AB=BC=CD=DAAB = BC = CD = DA and all angles are 90∘90^\circ.

To prove: (a) AC=BDAC = BD, (b) diagonals bisect each other, (c) they bisect at right angles.


(a) AC = BD:

In △ABC\triangle ABC and △BAD\triangle BAD:

  • AB=BAAB = BA (common)
  • BC=ADBC = AD (sides of a square)
  • ∠ABC=∠BAD=90∘\angle ABC = \angle BAD = 90^\circ

By SAS: △ABC≅△BAD\triangle ABC \cong \triangle BAD

∴AC=BD\therefore AC = BD (CPCT) ✓\checkmark


(b) Diagonals bisect each other:

Let diagonals AC and BD intersect at O.

In △AOB\triangle AOB and △COD\triangle COD:

  • AB=CDAB = CD (sides of a square)
  • ∠OAB=∠OCD\angle OAB = \angle OCD (alternate interior angles, AB∥CDAB \parallel CD)
  • ∠OBA=∠ODC\angle OBA = \angle ODC (alternate interior angles)

By AAS: △AOB≅△COD\triangle AOB \cong \triangle COD

∴OA=OC\therefore OA = OC and OB=ODOB = OD (CPCT)

Hence diagonals bisect each other. ✓\checkmark


(c) Diagonals bisect at right angles:

In △AOB\triangle AOB and △AOD\triangle AOD:

  • OB=ODOB = OD (proved above)
  • AB=ADAB = AD (sides of a square)
  • AO=AOAO = AO (common)

By SSS: △AOB≅△AOD\triangle AOB \cong \triangle AOD

∴∠AOB=∠AOD\therefore \angle AOB = \angle AOD (CPCT)

But ∠AOB+∠AOD=180∘\angle AOB + \angle AOD = 180^\circ (linear pair)

⇒2∠AOB=180∘⇒∠AOB=90∘\Rightarrow 2\angle AOB = 180^\circ \Rightarrow \angle AOB = 90^\circ

Hence diagonals bisect each other at right angles. ■\blacksquare

3Diagonal AC of a parallelogram ABCD bisects ∠A\angle A (see Fig. 8.11). Show that (i) it bisects ∠C\angle C also, (ii) ABCD is a rhombus.Show solution

Given: ABCD is a parallelogram in which diagonal AC bisects ∠A\angle A, i.e., ∠DAC=∠BAC\angle DAC = \angle BAC.


(i) AC bisects ∠C\angle C:

Since AB∥DCAB \parallel DC and ACAC is a transversal:
∠BAC=∠DCA(alternate interior angles)…(1)\angle BAC = \angle DCA \quad \text{(alternate interior angles)} \quad \ldots(1)

Since AD∥BCAD \parallel BC and ACAC is a transversal:
∠DAC=∠BCA(alternate interior angles)…(2)\angle DAC = \angle BCA \quad \text{(alternate interior angles)} \quad \ldots(2)

But ∠DAC=∠BAC\angle DAC = \angle BAC (given) …(3)\ldots(3)

From (1), (2) and (3):
∠DCA=∠BCA\angle DCA = \angle BCA

Therefore, AC bisects ∠C\angle C also. ✓\checkmark


(ii) ABCD is a rhombus:

In △ABC\triangle ABC:
∠BAC=∠BCA(from (1) and (3))\angle BAC = \angle BCA \quad \text{(from (1) and (3))}

⇒BC=AB\Rightarrow BC = AB (sides opposite equal angles are equal)

Since ABCD is a parallelogram, AB=CDAB = CD and BC=ADBC = AD.

Therefore AB=BC=CD=DAAB = BC = CD = DA.

Hence ABCD is a rhombus. ■\blacksquare

4ABCD is a rectangle in which diagonal AC bisects ∠A\angle A as well as ∠C\angle C. Show that: (i) ABCD is a square (ii) diagonal BD bisects ∠B\angle B as well as ∠D\angle D.Show solution

Given: ABCD is a rectangle; AC bisects ∠A\angle A and ∠C\angle C.

So ∠DAC=∠BAC=45∘\angle DAC = \angle BAC = 45^\circ and ∠DCA=∠BCA=45∘\angle DCA = \angle BCA = 45^\circ (since each angle of a rectangle is 90∘90^\circ).


(i) ABCD is a square:

In △ABC\triangle ABC:
∠BAC=45∘ and ∠BCA=45∘\angle BAC = 45^\circ \text{ and } \angle BCA = 45^\circ
⇒∠BAC=∠BCA\Rightarrow \angle BAC = \angle BCA
⇒BC=AB(sides opposite equal angles)\Rightarrow BC = AB \quad \text{(sides opposite equal angles)}

Since ABCD is a rectangle, AB=CDAB = CD and BC=ADBC = AD.

Therefore AB=BC=CD=DAAB = BC = CD = DA.

A rectangle with all sides equal is a square. ✓\checkmark


(ii) BD bisects ∠B\angle B and ∠D\angle D:

Since ABCD is a square, AB=BC=CD=DAAB = BC = CD = DA.

In △ABD\triangle ABD:
AB=AD⇒∠ABD=∠ADBAB = AD \Rightarrow \angle ABD = \angle ADB

Also ∠ABD+∠ADB=90∘\angle ABD + \angle ADB = 90^\circ (since ∠A=90∘\angle A = 90^\circ)
⇒∠ABD=∠ADB=45∘=12×90∘\Rightarrow \angle ABD = \angle ADB = 45^\circ = \frac{1}{2} \times 90^\circ

So BD bisects ∠B\angle B.

Similarly in △BCD\triangle BCD: BC=CD⇒∠CBD=∠CDB=45∘BC = CD \Rightarrow \angle CBD = \angle CDB = 45^\circ

So BD bisects ∠D\angle D also. ■\blacksquare

5In parallelogram ABCD, two points P and Q are taken on diagonal BD such that DP = BQ (see Fig. 8.12). Show that: (i) ΔAPD≅ΔCQB\Delta APD \cong \Delta CQB (ii) AP=CQAP = CQ (iii) ΔAQB≅ΔCPD\Delta AQB \cong \Delta CPD (iv) AQ=CPAQ = CP (v) APCQ is a parallelogram.Show solution

Given: ABCD is a parallelogram; P and Q are on diagonal BD such that DP=BQDP = BQ.


(i) △APD≅△CQB\triangle APD \cong \triangle CQB:

In △APD\triangle APD and △CQB\triangle CQB:

  • AD=CBAD = CB (opposite sides of parallelogram)
  • DP=BQDP = BQ (given)
  • ∠ADP=∠CBQ\angle ADP = \angle CBQ (alternate interior angles, since AD∥BCAD \parallel BC and BDBD is transversal)

By SAS: △APD≅△CQB\triangle APD \cong \triangle CQB ✓\checkmark


(ii) AP=CQAP = CQ:

From (i), AP=CQAP = CQ (CPCT) ✓\checkmark


(iii) △AQB≅△CPD\triangle AQB \cong \triangle CPD:

In △AQB\triangle AQB and △CPD\triangle CPD:

  • AB=CDAB = CD (opposite sides of parallelogram)
  • BQ=DPBQ = DP (given)
  • ∠ABQ=∠CDP\angle ABQ = \angle CDP (alternate interior angles, since AB∥CDAB \parallel CD and BDBD is transversal)

By SAS: △AQB≅△CPD\triangle AQB \cong \triangle CPD ✓\checkmark


(iv) AQ=CPAQ = CP:

From (iii), AQ=CPAQ = CP (CPCT) ✓\checkmark


(v) APCQ is a parallelogram:

From (ii): AP=CQAP = CQ
From (iv): AQ=CPAQ = CP

Since both pairs of opposite sides are equal, APCQ is a parallelogram. ■\blacksquare

6ABCD is a parallelogram and AP and CQ are perpendiculars from vertices A and C on diagonal BD (see Fig. 8.13). Show that (i) ΔAPB≅ΔCQD\Delta APB \cong \Delta CQD (ii) AP=CQAP = CQ.Show solution

Given: ABCD is a parallelogram; AP⊥BDAP \perp BD and CQ⊥BDCQ \perp BD.


(i) △APB≅△CQD\triangle APB \cong \triangle CQD:

In △APB\triangle APB and △CQD\triangle CQD:

  • ∠APB=∠CQD=90∘\angle APB = \angle CQD = 90^\circ (given, AP and CQ are perpendiculars)
  • AB=CDAB = CD (opposite sides of parallelogram)
  • ∠ABP=∠CDQ\angle ABP = \angle CDQ (alternate interior angles, since AB∥CDAB \parallel CD and BDBD is transversal)

By AAS: △APB≅△CQD\triangle APB \cong \triangle CQD ✓\checkmark


(ii) AP=CQAP = CQ:

From (i), AP=CQAP = CQ (CPCT) ■\blacksquare

7ABCD is a trapezium in which AB || CD and AD = BC (see Fig. 8.14). Show that (i) ∠A=∠B\angle A = \angle B (ii) ∠C=∠D\angle C = \angle D (iii) ΔABC≅ΔBAD\Delta ABC \cong \Delta BAD (iv) diagonal AC = diagonal BD.
[Hint: Extend AB and draw a line through C parallel to DA intersecting AB produced at E.]
Show solution

Given: ABCD is a trapezium with AB∥CDAB \parallel CD and AD=BCAD = BC.

Construction: Extend AB to E and draw CE∥DACE \parallel DA, meeting AB produced at E.


(i) ∠A=∠B\angle A = \angle B:

Since AD∥CEAD \parallel CE and AE∥DCAE \parallel DC (by construction and given), ADCE is a parallelogram.

∴AD=CE\therefore AD = CE (opposite sides of parallelogram)

But AD=BCAD = BC (given), so BC=CEBC = CE.

In △BCE\triangle BCE: BC=CE⇒∠CBE=∠CEBBC = CE \Rightarrow \angle CBE = \angle CEB (base angles of isosceles triangle)

∠CBE=∠ABC\angle CBE = \angle ABC (same angle)

Now, ∠DAB+∠CBE=180∘\angle DAB + \angle CBE = 180^\circ ... wait, let us use co-interior angles.

Since AD∥CEAD \parallel CE:
∠DAE+∠CEA=180∘(co-interior angles on same side of AE)\angle DAE + \angle CEA = 180^\circ \quad \text{(co-interior angles on same side of AE)}

In △BCE\triangle BCE, BC=CEBC = CE, so ∠CBE=∠CEB\angle CBE = \angle CEB.

∠ABC=180∘−∠CBE=180∘−∠CEB\angle ABC = 180^\circ - \angle CBE = 180^\circ - \angle CEB

Also ∠DAB+∠CEB=180∘\angle DAB + \angle CEB = 180^\circ (since ∠DAE=∠DAB\angle DAE = \angle DAB and ∠CEA=∠CEB\angle CEA = \angle CEB, co-interior angles with AD∥CEAD \parallel CE)

⇒∠DAB=180∘−∠CEB=∠ABC\Rightarrow \angle DAB = 180^\circ - \angle CEB = \angle ABC

∴∠A=∠B✓\therefore \angle A = \angle B \checkmark


(ii) ∠C=∠D\angle C = \angle D:

Since AB∥CDAB \parallel CD:
∠A+∠D=180∘and∠B+∠C=180∘\angle A + \angle D = 180^\circ \quad \text{and} \quad \angle B + \angle C = 180^\circ

Since ∠A=∠B\angle A = \angle B:
∠D=180∘−∠A=180∘−∠B=∠C\angle D = 180^\circ - \angle A = 180^\circ - \angle B = \angle C

∴∠C=∠D✓\therefore \angle C = \angle D \checkmark


(iii) △ABC≅△BAD\triangle ABC \cong \triangle BAD:

In △ABC\triangle ABC and △BAD\triangle BAD:

  • AB=BAAB = BA (common)
  • BC=ADBC = AD (given)
  • ∠ABC=∠BAD\angle ABC = \angle BAD (proved in (i))

By SAS: △ABC≅△BAD\triangle ABC \cong \triangle BAD ✓\checkmark


(iv) diagonal AC = diagonal BD:

From (iii): AC=BDAC = BD (CPCT) ■\blacksquare

Exercise 8.2

1ABCD is a quadrilateral in which P, Q, R and S are mid-points of the sides AB, BC, CD and DA (see Fig 8.20). AC is a diagonal. Show that: (i) SR∥ACSR \parallel AC and SR=12ACSR = \frac{1}{2}AC (ii) PQ=SRPQ = SR (iii) PQRS is a parallelogram.

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2ABCD is a rhombus and P, Q, R and S are the mid-points of the sides AB, BC, CD and DA respectively. Show that the quadrilateral PQRS is a rectangle.

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3ABCD is a rectangle and P, Q, R and S are mid-points of the sides AB, BC, CD and DA respectively. Show that the quadrilateral PQRS is a rhombus.

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4ABCD is a trapezium in which AB ∥\parallel DC, BD is a diagonal and E is the mid-point of AD. A line is drawn through E parallel to AB intersecting BC at F (see Fig. 8.21). Show that F is the mid-point of BC.

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5In a parallelogram ABCD, E and F are the mid-points of sides AB and CD respectively (see Fig. 8.22). Show that the line segments AF and EC trisect the diagonal BD.

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6ABC is a triangle right angled at C. A line through the mid-point M of hypotenuse AB and parallel to BC intersects AC at D. Show that (i) D is the mid-point of AC (ii) MD⊥ACMD \perp AC (iii) CM=MA=12ABCM = MA = \frac{1}{2}AB.

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Frequently Asked Questions

What are the important topics in Quadrilaterals for Madhya Pradesh Board Class 9 Mathematics?
Key topics in Quadrilaterals include Properties of Parallelograms, Mid-point Theorem, Special Quadrilaterals. Study these first, then practise questions on each for Class 9 exams.
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How should I revise Quadrilaterals for Class 9 exams?
Learn the core ideas first, then work through the 45 practice questions on Quadrilaterals. Revise definitions regularly and use flashcards for quick recall before the exam.

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