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posted by Joe on Tuesday, February 12, 2013 at 10:22pm.
Cos[x + (7 Pi)/4] + Cos[x - (7 Pi)/4] = 1
cos(x) cos(7pi/4) - sin(x) sin(7pi/4) + cos(x) cos(7pi/4) + sin(x) sin(7pi/4) = 1 2cos(x) cos(7pi/4) = 1 2cos(x) * 1/√2 = 1 cos(x) = 1/√2 x = pi/4 or 7pi/4
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