How to collect sin(t) or (sin(t)) ^2 coefficients in a expression

[j_verzani]

@Starpower_Power

using LinearAlgebra
using SymPy
@vars t x y 
m=[cos(t) -sin(t) 0;
      sin(t)  cos(t)  0; 
0 0 1];
a = inv(m)*[x;y;t]
b=a'*a

Your b is a vector, not a scalar. Try b[1] and the above should flow through. (In the first post, you have b=a'*a which dispatches to dot which should return a scalar in this usage.)