In Example 5, change 3/5 to 4/5 and then evaluate sin 2. Data from Example 5 If cos = 3/5 for a fourth-quadrant angle,

In Example 5, change 3/5 to 4/5 and then evaluate sin 2α.

Data from Example 5

If cos α = 3/5 for a fourth-quadrant angle, from Fig. 20.16(a) we see that sin α = −4/5. Thus,

In Fig. 20.16(b), angle 2α is shown to be in the third-quadrant, verifying the sign of the result. (cos α = 3 5, α = 307°,2α = 614°, which is a third-quadrant angle.)

sin2x = 2 sin a cosa = (-3)() 24 25 Eq. (20.21)