first, we are going to define variables as the following:
a = 0
a = π/2
n = 4 rectangles
Δx = [ b - a ] / n ------>Δx = [ π/2 - 0 ] / 4 = π/8
right endpoints :
sum( seq( 4 cos(x) * π/8 , x , 0+π/8 , π/2 , π/8 ) ) = 3.163065 underestimate
left endpoints:
sum( seq( 4 cos(x) * π/8 , x , 0 , π/2-(π/8) , π/8 ) ) = 4.733861 overestimate
the reason because the actual estimate by integral as the following:
π/2
∫ 4cos(x) dx = 4
0
