While reading in Signals book, here's what I read:(adsbygoogle = window.adsbygoogle || []).push({});

I=1/2 [(integral sign) cos(n+m)wt dt + (integral sing) cos (n-m)wt dt]

"Because cos wt executes one complete cycle during any interval of duration T, cos (n+m)wt executes (n+m) complete cycles during any interval of duration T. Therefore the first integral in the above equation, which represents the area under n+m complete cycles of a sinusoid, equals zero. The same argument shows that the second integral in the above equation is also zero, except when n=m. Hence I in the above equation is zero for all n is not equal to m."

My question is: How do these integrals become zero?

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# How does this integral become zero?

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