(a)
Given equation of tangent is $\frac{x}{a}\frac{\sqrt{3}}{2} + \frac{y}{b}\frac{1}{2} = 1$ and equation of tangent at the point $\left( a\cos\phi,\ b\sin\phi \right)$ on the ellipse is $\frac{x}{a}\cos\phi + \frac{y}{b}\sin\phi = 1$
Both are same
$\therefore\cos\phi = \frac{\sqrt{3}}{2},\ \sin\phi = \frac{1}{2} \Rightarrow \phi = \frac{\pi}{6}$