For the ellipse $b^2x^2+a^2y^2=a^2b^2$, the gradient of the tangent at $(a\cos\theta,\,b\sin\theta)$ is:
- $\frac{a\sin\theta}{b\cos\theta}$
- $\frac{b\cos\theta}{a\sin\theta}$
- $-\frac{a\sin\theta}{b\cos\theta}$
- $-\frac{b\cos\theta}{a\sin\theta}$
Show answer and solution
Correct answer: D. $-\frac{b\cos\theta}{a\sin\theta}$
1. Differentiate $b^2x^2+a^2y^2=a^2b^2$ implicitly: $2b^2x+2a^2y\dfrac{dy}{dx}=0$.
2. Rearranging gives $\dfrac{dy}{dx}=-\dfrac{b^2x}{a^2y}$.
3. Substitute the point $x=a\cos\theta$, $y=b\sin\theta$.
4. $\dfrac{dy}{dx}=-\dfrac{b^2(a\cos\theta)}{a^2(b\sin\theta)}=-\dfrac{b\cos\theta}{a\sin\theta}$.
5. Differentiating the parametric equations separately gives the same result. The form $-\dfrac{a\sin\theta}{b\cos\theta}$ is the normal's gradient, the negative reciprocal.
_Source: NECTA ACSEE 2023 Advanced Mathematics 142/2, Question 8: Coordinate Geometry II_