Practicing Success

Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Applications of Derivatives

Question:

The lengths of tangent, subtangent, normal and subnormal for the curve $y=x^2+x-1$ at (1, 1) are A, B, C and D respectively, then their increasing order is

Options:

B, D, A, C

B, A, C, D

A, B, C, D

B, A, D, C

Correct Answer:

B, A, D, C

Explanation:

The equation of the curve is $y=x^2+x-1$

∴  $\frac{d y}{d x}=2 x+1 \Rightarrow\left(\frac{d y}{d x}\right)_{(1,1)}=3$

Now, 

A = Length of the tangent at (1, 1)

$\Rightarrow A=\frac{\sqrt{1+\left(\frac{d y}{d x}\right)^2}}{\frac{d y}{d x}}=\frac{\sqrt{1+3^2}}{3}=\frac{\sqrt{10}}{3}$

B = Length of the subtangent at (1, 1)

$\Rightarrow B=\frac{y}{\frac{d x}{d y}}=\frac{1}{3}$

C = Length of the normal at (1, 1)

$\Rightarrow C =\sqrt{1+\left(\frac{d y}{d x}\right)^2}=\sqrt{1+3^2}=\sqrt{10}$

D = Length of the subnormal at (1, 1)

$\Rightarrow D =y \frac{d y}{d x}=1 \times 3=3$

Thus, we have B < A < D < C.