Question
Use a double-angle formula to find the exact value of cos2x when sinx = 3/5, where pi/2 < x < pi.
-(7/25) or 7/25?
-(7/25) or 7/25?
Answers
well, just keep track of the signs as you work it out.
cos2x = 1 - 2sin^2x = 1 - 2*9/25 = 7/25
Or, if you insist on doing it the hard way,
cos2x = cos^2x - sin^2x = 16/25 - 9/25 = 7/25
If you came up with -7/25, you need to remember your simple Algebra I!
cos2x = 1 - 2sin^2x = 1 - 2*9/25 = 7/25
Or, if you insist on doing it the hard way,
cos2x = cos^2x - sin^2x = 16/25 - 9/25 = 7/25
If you came up with -7/25, you need to remember your simple Algebra I!
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