Then you can use
This graph does not cross the x-axis as the maximum value of is 1.
The notation you asked about....
That graph crosses the x-axis every 90 degrees,
however it is symmetrical about the x-axis.
As you are integrating from 0 to 360 degrees,
we calculate 4 times the integral from 0 to 90 degrees.
Also we have negative f(t) for and