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Class 11 RD Sharma Solutions- Chapter 23 The Straight Lines- Exercise 23.17
  • Last Updated : 19 Jan, 2021
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Question 1. Prove that the area of the parallelogram formed by the lines

a1x + b1y + c1 = 0, a1x + b1y + d1 = 0, a2x + b2y + c2 = 0, a2x + b2y + d2 = 0 is \left|\frac{(d_1-c_1)(d_2-c_2)}{a_1b_2-a_2b_1}\right|  sq. units.

Deduce the condition for these lines to form a rhombus.

Solution:

Given:

The given lines are

a1x + b1y + c1 = 0 —(equation-1)

a1x + b1y + d1 = 0 —(equation-2)



a2x + b2y + c2 = 0 —(equation-3)

a2x + b2y + d2 = 0 —(equation-4)

Let us prove, the area of the parallelogram formed by the lines a1x + b1y + c1 = 0, a1x + b1y + d1 = 0, a2x + b2y + c2 = 0, a2x + b2y + d2 = 0 is  

\left|\frac{(d_1-c_1)(d_2-c_2)}{a_1b_2-a_2b_1}\right|  sq. units.

The area of the parallelogram formed by the lines a1x + b1y + c1 = 0, a1x + b1y + d1 = 0, a2x + b2y + c2 = 0 and a2x + b2y + d2 = 0 is given below:

Area  = \left|\frac{(c_1-d_1)(c_2-d_2)}{\begin{vmatrix}a_1&a_2\\b_1&b_2\end{vmatrix}}\right|

Since, \begin{vmatrix}a_1&a_2\\b_1&b_2\end{vmatrix} = a1b2 – a2b1

Therefore, Area = \left|\frac{(c_1-d_1)(c_2-d_2)}{a_1b_2-a_2b_1}\right|=\left|\frac{(d_1-c_1)(d_2-c_2)}{a_1b_2-a_2b_1}\right|



If the given parallelogram is a rhombus, then the distance between the pair of parallel lines is equal.

Therefore, \left|\frac{c_1-d_1}{\sqrt{a_1^2+b_1^2}}\right|=\left|\frac{c_2-d_2}{\sqrt{a_1^2+b_1^2}}\right|

Hence proved.

Question 2. Prove that the area of the parallelogram formed by the lines 3x – 4y + a = 0, 3x – 4y + 3a = 0, 4x – 3y – a = 0 and 4x – 3y – 2a = 0 is \frac{2a^2}{7}  sq. units.

Solution:

Given:

The given lines are

3x − 4y + a = 0 —(equation-1)

3x − 4y + 3a = 0 —(equation-2)

4x − 3y − a = 0 —(equation-3)

4x − 3y − 2a = 0 —(equation-4)

We have to prove, the area of the parallelogram formed by the lines 3x – 4y + a = 0, 3x – 4y + 3a = 0, 4x – 3y – a = 0 and 4x – 3y – 2a = 0 is \frac{2a^2}{7}  sq. units.

From above solution, we know that

Area of the parallelogram = \left|\frac{(c_1-d_1)(c_2-d_2)}{a_1b_2-a_2b_1}\right|

Area of the parallelogram =  \left|\frac{(a-3a)(2a-a)}{(-9+16)}\right|=\frac{2a^2}{7}  sq. units

Hence proved.

Question 3. Show that the diagonals of the parallelogram whose sides are lx + my + n = 0, lx + my + n’ = 0, mx + ly + n = 0 and mx + ly + n’ = 0 include an angle π/2.

Solution:

Given:

The given lines are

lx + my + n = 0 —(equation-1)

mx + ly + n’ = 0 —(equation-2)

lx + my + n’ = 0 —(equation-3)

mx + ly + n = 0 —(equation-4)

We have to prove, the diagonals of the parallelogram whose sides are lx + my + n = 0, lx + my + n’ = 0, mx + ly + n = 0 and mx + ly + n’ = 0 include an angle \frac{\pi}{2} .

By solving equation (1) and (2), we will get

B = \left(\frac{mn'-ln}{l^2-m^2}, \frac{mn-ln'}{l^2-m^2}\right)

Solving equation (2) and (3), we get 

C = \left(-\frac{n'}{m+1'}, -\frac{n'}{m+1}\right)

Solving equation (3) and (4), we get 

D = \left(\frac{mn-ln'}{l^2-m^2}, \frac{mn'-ln}{l^2-m^2}\right)

Solving equation (1) and (4), we get

A = \left(-\frac{n}{m+1'}, -\frac{n}{m+1}\right)  

Let m1 and m2 be the slope of AC and BD.

Now,

m1\frac{\frac{-n'}{m+1}+\frac{n}{m+1}}{\frac{-n'}{m+1}+\frac{n}{m+1}}=1

m2\frac{\frac{mn'-ln}{l^2-m^2}+\frac{mn-ln'}{l^2-m^2}}{\frac{mn-ln'}{l^2-m^2}+\frac{mn'-ln}{l^2-m^2}}=-1

Therefore, 

m1m2 = -1

Hence proved.

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