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M(t)=21cos(2π687t)+228M(t) = 21\cos\left(\dfrac{2\pi}{687}t\right) + 228M(t)=21cos(6872πt)+228M, left parenthesis, t, right parenthesis, equals, 21, cosine, left parenthesis, start fraction, 2, pi, divided by, 687, end fraction, t, right parenthesis, plus, 228