(a)
Angle between the lines ${ax}^{2} + 2hxy + {by}^{2} = 0$ is
$\tan\theta = \left| \frac{2\sqrt{h^{2} - ab}}{a + b} \right|$
For $x^{2} + 2xy\sec\theta + y^{2} = 0$
$h = \sec\theta,\ a = b = 1$
$\therefore\ \tan\phi = \left| \frac{2\sqrt{\sec^{2}\theta - 1}}{1 + 1} \right|$
$= \frac{2\tan\theta}{2} = \tan\theta$
$\therefore$ Angle between $x^{2} + 2xy\sec\theta + y^{2} = 0\ is\ \theta$