Question

--Original Question:
A hollow cylinder with outer radius r_0 and inner radius r_i is loaded by a uniformly distributed load q across its length L. The beam is supported by a roller at its left end and a pin L/5 from its right end.

What is the maximum bending moment in the beam? Express your answer in terms of q and L.

--Worked out solution:
www.jiskha.com/questions/1822768/a-hollow-cylinder-with-outer-radius-r-0-and-inner-radius-r-i-is-loaded-by-a-uniformly

--I'm just trying to figure out the steps: Why does the derivative of x become x? and how does a 2 show up in the denominator?

Thanks in advance for any further guidance someone can provide me.