Chapter 5.1, Problem 4E

In the following exercises, estimate the volume of the solid under the surface z= f(x. y) and above the rectangular

legion R by using a Riemann sum with m = n = 2 and the sample points to be the lower left corners of the subrectangles of the partition.

4. $f\left(x,y\right)=\cos x+\cos y,R=\left[0,\pi \right]\times \left[0,\frac{\pi }{2}\right]$