If sin α = 4/5, then cos(2α) is?
- - 7/25
- 9/25
- 2/25
- 23/25
Explanation
sin α = 4/5 ⇒ cos α = 3/5 (from Pythagoras)
cos(2α) = 2cos²α − 1 = 2*(9/25) − 1 = 18/25 − 25/25 = −7/25
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