a = x_0

b = x_n

x_1

x_2

Δx

f(x) = x
a = LowerSum(f, 2, 6, 4)
b = LowerSum(f, 1.5, 6.5, 8)
A = (1.5, 0)
B = (6.5, 0)
c = UpperSum(f, 2, 6, 4)
e = UpperSum(f, 1.5, 6.5, 8)