How can you prove that the function f(n) is big theta of g(n)?

To prove that the function f(n) is big theta of g(n), you need to show that there exist positive constants c1, c2, and n0 such that for all n greater than or equal to n0, c1g(n) f(n) c2g(n). This means that f(n) grows at the same rate as g(n) within a constant factor for sufficiently large n.