Question 1105468
1. cos 2pix = 1
cos(2pi*x) = 1
2pi*x = 0 + 2n*pi --- n = integer
x = 0 + n  --- n = integer
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2. cos(2pi*x) = -1
2pi*x = pi + 2n*pi --- n = integer
x = 1/2 + n --- n = integer