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sin2 + cos2 = 1
Therefore the sine and cosine functions are periodic with period 2.
sin(x + 2) = sinx
cos(x + 2) = cosx
sin(-) = - sin(), the sine function is odd.
cos(-) = cos(), the cosine function is even.
sin(x+y) = sinx cosy + cosx siny
cos(x+y) = cosx cosy - sinxsiny