Posted by kk on Monday, April 2, 2012 at 12:47pm.
cos^2 θ/2 = (1 + cosθ)/2
cos^2 22.5° = (1 + cos 45°)/2
= (1 + √2/2)/2
= (2 + √2)/4
hmmph
It would help to notice that
2cos^2 θ - 1 = cos 2θ
so what you have is just cos 45° = 1/√2
note that 2(2+√2)/4 - 1 = 1+√2/2-1 = √2/2 = 1/√2
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