The function is  on interval is .

Number of rectangles are .

The sum of all inscribed rectangle is lower sum.

, where 

The sum of all circumscribed rectangle is upper sum.

, where 

Width .

Find lower Sum.

.

Area of lower sum is .

Apply summation formula  .

Apply summation formula .

In this case .

.

Find upper Sum.

.

Area of higher sum is .

.

The function is  on interval  is decreasing.

.

.

Verification:

.