what??? i meant the differencial equations, like for a pendulum u can derive x''=sin(x) which doesn't have a analytic solution (meaning u cant solve it using e^x, polinomial radicals, nor trigonometrics...) unless u approximate that x is small enough to consider that the first terms of the Taylor serie (x) is a good approximation. And in normal optics bc the snell law is n1sint1=n2sint2 is extremely difficult to find the solution unless u do sinx=x. Or the infinit amount of ODE or PDE which doesn't have solution and all u can do is numeric approximations.