If Tana = 1 / 2, then what is cos (2a + π / 2), the answer is 45 degrees,

cos(2a+π/2)= -sin2a= -2sinacosa= -2sinacosa/(sin²a+cos²a)= -2tana/(tan²a+1)= -4/5.
The answer is - 4 / 5, not an angle!

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