if sine is -3/5 what is cosine?
$$cos\theta = \pm 4/5$$
sin is neg in the third and forth quadrant
cos is neg in the 3rd but pos in the 4th
cos = sqrt(1-sin^2)
cos = sqrt(1-(-3/5)^2)
cos = sqrt(1-9/25)
cos = sqrt(16/25)
cos = 4/5
