Question:
The value of $\cos ^{2} 17^{\circ}-\sin ^{2} 73^{\circ}$ is
(a) 1
(b) $\frac{1}{3}$
(c) 0
(d) $-1$
Solution:
We have:
$\cos ^{2} 17^{\circ}-\sin ^{2} 73^{\circ}$
$=\cos ^{2}\left(90^{\circ}-73^{\circ}\right)-\sin ^{2} 73^{\circ}$
$=\sin ^{2} 73^{\circ}-\sin ^{2} 73^{\circ}$
$=0$
Hence the correct option is $(c)$