### Home > A2C > Chapter 12 > Lesson 12.1.1 > Problem12-15

12-15.

Use the symmetry of the unit circle and the knowledge that $\operatorname { cos } \frac { \pi } { 4 } = \frac { \sqrt { 2 } } { 2 }$ to write three other true statements using cosine.

See problem 11-76 for help.

$\text{cos}\left(\pi-\frac{\pi}{4}\right)= \text{cos } \frac{3\pi}{4}=-\frac{\sqrt{2}}{2}$