A heavy rope, 40 ft long, weighs 0.5 lb/ft and hangs over the edge of a building 90 ft high.
(b) How much work W is done in pulling half the rope to the top of the building?
Show how to approximate the required work by a Riemann sum. (Enter xi* as xi.)

I used integration to get W=0.5*integrate*(40-x)dx(from x=0 to 20) and I got the final answer to be 300 ft/lb which is correct.

But the problem is asking me to write the work as a Riemann sum and as an integral and for some reason it will not except what I put.

Can anyone help me?! This is for WebAssign by the way.

Question

A heavy rope, 40 ft long, weighs 0.5 lb/ft and hangs over the edge of a building 90 ft high.
(b) How much work W is done in pulling half the rope to the top of the building?
Show how to approximate the required work by a Riemann sum. (Enter xi* as xi.)

I used integration to get W=0.5*integrate*(40-x)dx(from x=0 to 20) and I got the final answer to be 300 ft/lb which is correct.

But the problem is asking me to write the work as a Riemann sum and as an integral and for some reason it will not except what I put.

Can anyone help me?! This is for WebAssign by the way.