Question

Use the following to find the exact value of sin (theta/2). cos(theta)=(-4/5). Theta is in quadrant 3.

Answer 1

If theta is in QIII, then the x-value is negative. so the -4 is on the x-axis. Since the hypotenuse is 5, the other side is positive 3. This given, you can use the half-angle identity for sine. You know that cosine is -4/5 and sin is 3/5. So can you solve it from there?

Answer 2

Wow...I've gotten it! Big blonde moment!