Evaluate `int_(0)^(2pi)|cosx|dx`

Explanation

Let's break the integral as below :
`int_(0)^(2pi)|cosx|dx=int_(0)^((pi)/(2))|cosx|dx+int_(0)^((3pi)/(2))|cosx|dx+int_((3pi)/(2))^(2pi)|cosx|dx`
`=int_(0)^((pi)/(2))cosxdx-int_((pi)/(2))^((3pi)/(2))cosxdx+int_((3pi)/(2))^(2pi)cosxdx`
(We can found out the intervals in which `cosx` in positive or `cosx` is negative)
`=[sinx]_(0)^(pi//2)-[sinx]_(pi//2)^(3pi//2)+[sinx]_(3pi//2)^(2pi)`
`=(1-0)-[-1-1]+[0-(-1)]`
`=1+2+1`
`=4`