### Home > CCA2 > Chapter 12 > Lesson 12.2.1 > Problem 12-100

12-100.

If *π* ≤ *θ* ≤ *θ* = −*θ*. Use what you know about the Pythagorean Identity and the unit circle to get started. Homework Help ✎

Remember that the Pythagorean Identity is sin²(*θ*) + cos²(*θ*) = 1.

Try substituting the given value of sin(*θ*) into the equation, then solve for cos(*θ*).

(−^{3}/_{5})^{2} + cos^{2}(*θ*) = 1

cos^{2}(*θ*) = ^{16}/_{25}

Take the square root of both sides to solve for cos (*θ*).

Now use the unit circle to decide what the correct sign of the answer is.