Question #70b87

1 Answer
May 13, 2017

If Quadrant I: 4/5
If Quadrant II: -4/5

Explanation:

Use Pythagorean's Identity: sin^2x+cos^2x=1

By substitution, we get:
(3/5)^2+cos^2a=1

Subtract (3/5)^2 from both sides:
cos^2a=1-(3/5)^2
cos^2a=1-9/25
cos^2a=16/25

Now, we take the square root of both sides:
cosa=+-sqrt(16/25)
cosa=+-4/5

Therefore, if angleA is in Quadrant I, cosa=4/5
if angleA is in Quadrant II, cosa=-4/5