Question

What is the foot of the normal if the straight line x + y + 7 = 0 is normal to the hyperbola 3x^2 – 4y^2 = 12 whose normal is at the point (x1, y1)?

(a) (4, 3)

(b) (-4, 3)

(c) (4, -3)

(d) (-4, -3)

The question was posed to me during an online exam.

This intriguing question comes from Calculus Application topic in portion Application of Calculus of Mathematics – Class 12

Answer 1

Right option is (d) (-4, -3)

The explanation is: Equation of the given hyperbola is, 3x^2 – 4y^2 = 12 ……….(1)

Differentiating both sides of (1) with respect to y we get,

3*2x(dy/dx) – 4*(2y) = 0

Or dx/dy = 4y/3x

Therefore, the equation of the normal to the hyperbola (1) at the point (x1, y1) on it is,

y – y1 = -[dx/dy](x1, y1) (x – x1) = -4y1/3x1(x – x1)

Or 3x1y + 4y1x – 7x1y1 = 0

Now, if possible, let us assume that the straight line

x + y + 7 = 0 ………..(2)

This line is normal to the hyperbola (1) at the point (x1, y1). Then, the equation (2) and (3) must be identical. Hence, we have,

3x1/1 = 4y1/1 = -7x1y1/7

So, x1 = -4 and y1 = -3

Now, 3x1^2 – 4y1^2 = 3(-4)^2 – 4(-3)^2 = 12

This shows the point (-4, -3) lies on the hyperbola (1).

So, it’s the normal to the hyperbola.

Thus, it is evident that the straight line (3) is normal to the hyperbola (1); the co-ordinate foot is (-4, -3).