Question 622497
find the exact values of: 
sin2u 
given: sinu= -4/5 and pi < u 3pi/2

sinu=-4/5 in quadrants III and IV where sin<0
since quadrant IV is not in the domain,(&#960;,3&#960;/2), reference angle is in quadrant III
sinu=-4/5=opposite side/hypotenuse
adjacent side=3 since you are working with a 3-4-5 right triangle
cosu=-3/5 in quadrants III where cos<0
sin 2u=2sinucosu
=2*-4/5*-3/5
=24/25