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)?

Category: QuestionsWhat 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)?
Editor">Editor Staff asked 11 months ago

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
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1 Answers
Editor">Editor Staff answered 11 months ago

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) = -4y1/3×1(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,
 
3×1/1 = 4y1/1 = -7x1y1/7
 
So, x1 = -4 and y1 = -3
 
Now, 3×1^2 – 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).