need help proving big omega in question 2 and 4

Prove that 5n^2 - 100 n elementof O(n^2) Prove that n^2 elementof Ohm(5n^2 - 100n) Prove that sigma^n_i = 1 i elementof O(n^2) Prove that sigma^n_i = 1 i elementof Ohm(n^2) Prove that sigma^n_i = 1 i^2 elementof O(n^3) Prove that sigma^n_i = 1 i^2 elementof (n^3)