Answer:
-4/5
Step-by-step explanation:
sin theta = opp/ hyp
sin theta = -3 /5
Using the Pythagorean theorem
opp ^2 + adj ^2 = hyp ^2
(-3) ^2 + adj ^2 = 5^2
9+ adj ^2 = 25
adj ^2 = 25 - 9
adj ^2 = 16
Taking the square root of each side
adj = ±4
Since we are in the 3rd quadrant sin and cos are both negative so adj must be negative
adj = -4
cos theta = adj / hyp
cos theta = -4/5
