find sin if cos(2è)=(4)/(5) and theta terminates in quadrant 1

Answer 1

cos 2Ø = 1 - 2sin^2 Ø
2sin^2 Ø = 1 - cos 2Ø = 1 - 4/5
sin^2 Ø = 1/10
sin Ø = 1/√10