cosa = cos^2(a/2) - sin^2(a/2)
sin^2(a/2) = 1 - cos^2(a/2) = 1 - 1/5 = 4/5
cosa = 1/5 - 4/5 = -3/5
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cosa = cos^2(a/2) - sin^2(a/2)
sin^2(a/2) = 1 - cos^2(a/2) = 1 - 1/5 = 4/5
cosa = 1/5 - 4/5 = -3/5