n = 4
values of intervals
First, like we have said in the other post, use radians lol.
Also, although the partition is 0, 0.25, 0.5, 0.75, 1, midpoint rule requires you to find f(0.125), f(0.375), f(0.625) and f(0.875).
Add them up and multiply by the partition width - 0.25.