SOLUTION: let cos x = 1/sqrt(10) with x in quadrant IV. find cos 2x.

Algebra ->  Trigonometry-basics -> SOLUTION: let cos x = 1/sqrt(10) with x in quadrant IV. find cos 2x.      Log On


   

Question 931556: let cos x = 1/sqrt(10) with x in quadrant IV. find cos 2x.
Answer by tommyt3rd(5050) About Me  (Show Source):
You can put this solution on YOUR website!
cos%282x%29=2cos%28x%29%5E2-1
so we can write:
2%281%2Fsqrt%2810%29%29%5E2-1=1%2F5-1=-4%2F5

but cosine is positive in Quad IV, so...

x=4%2F5