1) split the range in three identical invervals of size [6 - 0] / 3 = 2
2) form three rectangles
2a) First rectangle: base 2, height f(2) = 2^2 + 1 = 5
area 1 = base height = 2 5 = 10
2b) second rectangle: base 2, height f(2+2) = 4^2 + 1 = 17
area 2 = 2 17 = 34
2c) third rectangle: base 2 height f(4+2) = 6^2 + 1 = 37
area 3 = 237 = 74
3) total area = area 1 + area 2 + area 3 = 10 + 34 + 74 = 118
