# Class 9 RD Sharma Solutions – Chapter 14 Quadrilaterals- Exercise 14.4 | Set 1

### Question 1. In Î”ABC, D, E and F are, respectively the mid points of BC, CA and AB. If the lengths of sides AB, BC and CA are 7cm, 8 cm, and 9 cm, respectively, find the perimeter of Î”DEF.

Solution:

From the question it is given that

AB = 7 cm, BC = 8 cm, AC = 9 cm

Find: the perimeter of Î”DEF

In âˆ†ABC,

D, E and F are the mid points of BC, CA and AB.

So, by midpoint theorem

EF = 1/2 BC,

DF = 1/2 AC and DE = 1/2 AB

Now, we find the perimeter of Î”DEF

So, Perimeter of âˆ†DEF = DE + EF + DF

= (1/2) AB + (1/2) BC + (1/2) AC

= 1/2 (AB + BC + AC)

= 1/2(7 + 8 + 9)

= 1/2 (24)

= 12 cm

Hence, the perimeter of Î”DEF is 12 cm

### Question 2. In a Î”ABC, âˆ A = 50Â°, âˆ B = 60Â° and âˆ C = 70Â°. Find the measures of the angles of the triangle formed by joining the mid-points of the sides of this triangle.

Solution:

From the question it is given that

âˆ A = 50Â°, âˆ B = 60Â° and âˆ C = 70Â°

In Î”ABC,

D, E, and F are mid points of AB, BC, and AC

So from Midpoint Theorem

DE âˆ¥ AC, DE = (1/2) AC

DE = (1/2) AC = CF

DE âˆ¥ AC, DE = CF [Proved above]

Hence, DECF is a parallelogram.

So, âˆ C = âˆ D = 70Â°  [Because the opposite sides of a parallelogram are equal]

Similarly,

BEFD is a parallelogram,

So, âˆ B = âˆ F = 60Â°

ADEF is a parallelogram,

So, âˆ A = âˆ E = 50Â°

Hence, the angles of Î”DEF are

âˆ D = 70Â°, âˆ E = 50Â°, âˆ F = 60Â°

### Question 3. In a triangle, P, Q and R are the mid points of sides BC, CA and AB respectively. If AC = 21cm, BC = 29 cm and AB = 30 cm, find the perimeter of the quadrilateral ARPQ.

Solution:

From the question it is given that

AC = 21cm, BC = 29 cm and AB = 30 cm

In Î”ABC,

R and P are mid points of AB and BC

So from Midpoint Theorem

RP âˆ¥ AC, RP = (1/2) AC

RP âˆ¥ AQ, RP = AQ [Because the opposite sides of a parallelogram

are equal and parallel to each other]

Hence, AQPR is a parallelogram

Now,

AR = (1/2) AB = 1/2 x 30 = 15 cm

So, AR = QP = 15 cm [Because the opposite sides of a parallelogram are equal]

Similarly,

RP = (1/2) AC =1/2 x 21 = 10.5 cm

So, RP = AQ = 10.5 cm [Because the opposite sides of a parallelogram are equal]

Now we find the perimeter of the quadrilateral ARPQ

So, Perimeter of ARPQ = AR + QP + RP + AQ

= 15 + 15 + 10.5 + 10.5

= 51 cm

Hence, the perimeter of the ARPQ = 51cm

### Question 4. In a Î”ABC median AD is produced to x such that AD = DX. Prove that ABXC is a parallelogram.

Solution:

From the question it is given that

BD = DC

To prove: Prove that ABXC is a parallelogram.

Proof: Now,

In a quadrilateral ABXC, we have

AD = DX               [Given]

BD = DC               [Given]

So, diagonals AX and BC bisect each other.

Hence, ABXC is a parallelogram

### Question 5. In a Î”ABC, E and F are the mid-points of AC and AB respectively. The altitude AP to BC intersects FE at Q. Prove that AQ = QP.

Solution:

In Î”ABC

E and F are mid points of AB and AC

So from midpoint theorem

EF âˆ¥ FE, (1/2) BC = FE

Similarly,

In Î”ABP

F is the mid-point of AB

So, from mid-point theorem

So, FQ âˆ¥ BP [since EF âˆ¥ BP]

Q is the mid-point of AP

Hence AQ = QP

### Question 6. In a Î”ABC, BM and CN are perpendiculars from B and C respectively on any line passing through A. If L is the mid-point of BC, prove that ML = NL.

Solution:

In Î”BLM and Î”CLN

âˆ BML = âˆ CNL = 90Â°

BL = CL         [Because L is the mid-point of BC]

âˆ MLB = âˆ NLC      [Because vertically opposite angle]

So, Î”BLM â‰… Î”CLN

Hence, by corresponding parts of congruent triangles

LM = LN

### (ii) The area of Î”ADE.

Solution:

From the question it is given that

AB = 9 cm, AC = 15 cm, âˆ B = 90Â°

D, E are the mid-points of AB and AC

In Î”ABC,

Using Pythagoras theorem

AC2 = AB2 + BC

= 152 = 92 + BC2

= BC2 = 225 â€“ 81 = 144

BC = 12

Similarly,

AD = DB = AB/2 = 9/2 = 4.5 cm     [D is the midâˆ’point of AB]

D and E are mid-points of AB and AC

So, from mid-point theorem

DE âˆ¥ BC â‡’ DE = BC/2

Now, we find the area of Î”ADE

So, Area = 1/2 x AD x DE

= 1/2 x 4.5 x 6

= 13.5

Hence, the area of Î”ADE is 13.5 cm2

### Question 8. In figure, M, N and P are mid-points of AB, AC and BC respectively. If MN = 3 cm, NP = 3.5 cm and MP = 2.5 cm, calculate BC, AB and AC.

Solution:

From the question it is given that

MN = 3 cm, NP = 3.5 cm and MP = 2.5 cm.

Find: the value of BC, AB and AC

In Î”ABC

M and N are mid-points of AB and AC

So, from mid-point theorem

MN = (1/2) BC, MN âˆ¥ BC

= 3 = (1/2) BC

= 3 x 2 = BC

= BC = 6 cm

Similarly,

AC = 2MP = 2 (2.5) = 5 cm

AB = 2 NP = 2 (3.5) = 7 cm

Hence, values of BC, AB, and AC are 6 cm,7 cm, and 5 cm

### Question 9. ABC is a triangle and through A, B, C lines are drawn parallel to BC, CA and AB respectively intersecting at P, Q and R. Prove that the perimeter of Î”PQR is double the perimeter of Î”ABC.

Solution:

To prove: Perimeter of Î”PQR is double the perimeter of Î”ABC.

Proof:

In Î”ABC

It is given that the Î”ABC pass through A, B, C lines are drawn

parallel to BC, CA and AB and intersecting at P, Q and R.

So, ABCQ and ARBC are parallelograms.

BC = AQ and BC = AR          [ Because the opposite sides of a parallelogram are equal]

= AQ = AR

= A is the mid-point of QR

Now we know that,

B and C are the mid points of PR and PQ

So, from mid-point theorem

AB = (1/2) PQ, BC = (1/2) QR, CA = (1/2) PR

= PQ = 2AB, QR = 2BC and PR = 2CA

= PQ + QR + RP = 2 (AB + BC + CA)

Perimeter of Î”PQR = 2 (perimeter of Î”ABC)

Hence proved.

### Question 10. In figure, BEâŠ¥ AC, AD is any line from A to BC intersecting BE in H. P, Q and R are respectively the mid-points of AH, AB, and BC. Prove that âˆ PQR = 90Â°

Solution:

From the question it is given that

BE âŠ¥ AC and P, Q and R are the mid-point of AH, AB and BC.

To prove: âˆ PQR = 90Â°

Proof:

In Î”ABC,

Q and R are mid-points of AB and BC

So, from mid-point theorem

QR âˆ¥ AC     ….. (i)

In Î”ABH,

Q and P are the mid-points of AB and AH

So, from mid-point theorem

QP âˆ¥ BH   ….. (ii)

But, BEâŠ¥AC

So, from eq(i) and (ii) we have,

QPâŠ¥QR

âˆ PQR = 90Â°

Hence Proved

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