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Prove that cos (π/4 + x) + cos (π/4 – x) = √2 cos x
cos (π/4 – x) cos (π/4 – y) – sin (π/4 – x) sin (π/4 – y) = sin (x + y)
sin (n + 1)x sin (n + 2)x + cos (n + 1)x cos (n + 2)x = cos x
cos (3π/4 + x) – cos (3π/4 – x) = –√2 sin x
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