1. ## Can someone help me please... (Integral/Diff. Calculus)

I was reviewing my lessons on our "Strength of Materials" and I came up with this problem.
I get stucked on this thing.

How did the final equation/formula came up?
I'm kinda curious.

help me please!! can you explain it to me briefly? and as simple as possible..

2. $\frac{(0.25)(20\pi)^2}{JG}= \frac{1600\pi^2}{JG}$

I see that there is no "JG" in the last formula. Presumably they have been replaced by some other formula. To get that final expression you must have $JG= 512\pi (0.20)^4(12x 10^6)$.

3. that's not what I meant.

I mean. can you explain to me, step by step (if you could) like how did they derive the equation, then integrate. and what are the derivatives that were used if there are.

btw, it's the formula for Torsion in strngth of materials,
theta = angle
J = torsion constant for the section
G = Modulus of elesticity
T = Torsion
L = Length ....