## Trigonometry (11th Edition) Clone

$\frac{\sqrt2}{2}$
RECALL: In a unit circle, $\cos{s} = x$ (Refer to Figure 13 on page 111) The angle $\frac{7\pi}{4}$ intersects the unit circle at the point $(\frac{\sqrt2}{2}, -\frac{\sqrt2}{2})$. This point has $x=\frac{\sqrt2}{2}$ and $y=-\frac{\sqrt2}{2}$. Since $\cos{s}=x$, then $\cos{\frac{7\pi}{4}} =\frac{\sqrt2}{2}$