What is the value of sin(x) if cos(x) is 4/5 and the angle is in the first quadrant?

3/5

Using the Pythagorean identity sin^2 + cos^2 = 1, we find sin^2 + (16/25) = 1, so sin^2 = 9/25. In the first quadrant, the sine is positive, so it is 3/5.