In Problem 9.12 of Griffiths Introduction to Electrodynamics, 4th edition (Problem 9.11 3rd edition), in the problem, he says that one can calculate the average energy density and Poynting vector as
using the formula
I don't really understand how to do...
In Griffiths Quantum Mechanics 2nd edition, in Chapter 8 he calculates the following integral on page 323
and he gets
I disagree with this result, I think the integral should be
Maybe somebody can explain why I am wrong? Also, from equation 8.24 to 8.25, he makes the assumption that...