I would like to know how $\frac T2$ came for time period in the image below.Because I think the frequncy is 1 so time period should be 1.Is that correct? or can anyone help me..
The article from where I got this image is here
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Note that the second half of the period of the sinusoid is simply equal to the first half times $-1$. Since the same is true for the square wave, the multiplication of the two is the same in both intervals $[0,T/2]$ and $[T/2,T]$. That's why the integral over the whole period equals twice the integral over the first half of the period.