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If, A normal is drawn at a point P(x, y) of a curve. It meets the x-axis at Q. If PQ is of constant length k. What kind of curve is passing through (0, k)?(a) Parabola(b) Hyperbola(c) Ellipse(d) CircleI had been asked this question in my homework.This intriguing question comes from Linear First Order Differential Equations topic in portion Differential Equations of Mathematics – Class 12

Answer»

The correct option is (d) Circle

The explanation is: Equation of the normal at a point P(x, y) is given by

Y – y = -1/(dy/dx)(X – x) ….(1)

Let the point Q at the x-axis be (x1 , 0).

From (1), we get

y(dy/dx) = x1 – x….(2)

Now, giving that PQ^2 = k^2

Or, x1 – x + y^2 = k^2

=>y(dy/dx) = ± √(k^2 – y^2)….(3)

(3) is the required differential equation for such curves,

Now solving (3) we get,

∫-dy/√(k^2 – y^2) = ∫-dx

Or, x^2 + y^2 = k^2 passes through (0, k)

Thus, it is a circle.



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