First time here? Check out the FAQ!
answered 2013-03-10 23:38:43 +0100
It is just a typo. Try the code written below
var('x1, x2, y1, y2, L, z, r, k') integral(cos(x1/L)*exp(-i*(k/z)*(x1*x2 + y1*y2)), x1, -L/2, L/2)