Practicing Success

Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Applications of Derivatives

Question:

The normal to the curve $x^2+2 x y-3 y^2=0$, at (1, 1) :

Options:

meets the curve again in the third quadrant

meets the curve again the fourth quadrant

does not meet the curve again

meets the curve again in the second quadrant

Correct Answer:

meets the curve again the fourth quadrant

Explanation:

The equation of the curve is

$x^2+2 x y-3 y^2=0$                ......(i)

Differentiating with respect to x, we get

$2 x+2 y+2 x \frac{d y}{d x}-6 y \frac{d y}{d x}=0 \Rightarrow \frac{d y}{d x}=\frac{x+y}{3 y-x} \Rightarrow\left(\frac{d y}{d x}\right)_{(1,1)}=1$

The equation of the normal at (1, 1) is

$y-1=-1(x-1)$ or, $x+y=2$                   ......(ii)

The x-coordinates of the points of intersection of (i) and (ii) are given by

$x^2+2 x(2-x)-3(2-x)^2=0$

$\Rightarrow 4 x^2-16 x+12=0$

$\Rightarrow x^2-4 x+3=0 \Rightarrow(x-3)(x-1)=0 \Rightarrow x=1,3$

Now, $x=1$ and $x+y=2 \Rightarrow y=1$

$x=3$ and $x+y=2 \Rightarrow y=-1$

Hence, the normal to the curve meets it again at $(3,-1)$ which lies in the fourth quadrant.