SOLUTION: find the exact value: sin (theta) = 4/5 lies in quadrant 1 , find cos 2(theta)

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Question 613275: find the exact value:
sin (theta) = 4/5 lies in quadrant 1 , find cos 2(theta)
Answer by jsmallt9(3758) About Me  (Show Source):
You can put this solution on YOUR website!
There are three variations of the cos%282theta%29 formula:
  • cos%282theta%29+=+cos%5E2%28theta%29-sin%5E2%28theta%29
  • cos%282theta%29+=+2cos%5E2%28theta%29-1
  • cos%282theta%29+=+1+-2sin%5E2%28theta%29
Any of these can be used. But since we only know sin%28theta%29 at the moment, it makes sense to use the third one: