在SymPy中集成勒让德多项式并将这些积分用作系数

I am trying to make a simple example in SymPy to compute some coefficients and then use them in a sum of legendre polynomials. Finally plot it. Very simple but can not make it work. I want to use it for my electromagnetism course. I get errors in both the attempts below:I am trying to make a simple example in SymPy t