Question 690368
use a double angle formulas to find the exact value of cos2u when sinu=3/5, where pi/2<u<pi
..
cos2u=1-2sin^u
=1-2*(3/5)^2
=1-18/25
=7/25
..
calculator check:
u=sin^-1(3/5)&#8776;2.498 (in quadrant II)
2u&#8776;4.9961 (in quadrant IV where cos>0)
cos2u=cos(4.9961)&#8776;0.2799..
7/25=0.28