If π ≤ θ ≤
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 + cos2(θ) = 1
cos2(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.