Question 828965
Find The value of cos2x if cotx=12/5, where pi < x <3pi/2
cotx=12/5(working with a (5-12-13) reference right triangle in quadrant III in which sin<0, cos<0
sinx=-5/13
cosx=-12/13
..
cos2x=cos^2x-sin^2x=144/169-25/169=119/169
..
Calculator check:
cosx=-12/13 in quadrant III
x&#8776;202.62
2x&#8776;405.24&#730;
cos2x&#8776;cos(405.24)&#8776;0.7041..
Exact value as calculated=119/169&#8776;0.7041..