SOLUTION: if sinx= -4/5 on the interval (pi , 3pi/2) what is the exact value of 2x

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Question 870529: if sinx= -4/5 on the interval (pi , 3pi/2) what is the exact value of 2x
Answer by lwsshak3(11628) About Me  (Show Source):
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if sinx= -4/5 on the interval (pi , 3pi/2) what is the exact value of 2x
you are working with a (3-4-5) reference right triangle in quadrant III
cosx=-3/5
cos 2x=cos^2x-sin^2x=(-3/5)^2-(4/5)^2=9/25-16/25=-7/25
cos 2x=-7/25
2x≈106.26˚