FEM Jacobi Matrix with variables

[trasor]

Hello everyone,

i have a question regarding variables in Jacobi-Matrices from isoparametric elements in 2D. I know that the entries are obtained by differntiating the shape functions. In my case however, variables such as xi, eta are left in the jacobi matrix. Since i thought the Matrix for one element should be always the same, i am not sure know which xi,eta values i should insert there.

Thanks in advance