Use the unit circle.

$\frac{\sqrt{2}}{2}$

See part (a).

$\text{Remember tan}\theta=\frac{\text{sin}\theta}{\text{cos}\theta}$

Where on the unit circle does sinθ = cosθ?

$\theta=\frac{\pi}{4},\,\frac{5\pi}{4}$

See part (c).