Given sin2a = 24 / 25, a ∈ (0, π / 2), then cos (a + π / 2) =?
It's a lot of calculation to use the solution of equations! Look at this solution!
∵sin2a=24/25.
∴cos2a=[√1-(24/25)∧2]=±7/25
And cos2a = 1-2sin Λ 2A
So sin ∧ 2A = (1-cos2a) / 2 = 16 / 25 or 9 / 25
∵ a ∈ (0, π / 2), so Sina = - 4 / 5 or - 3 / 5
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