Can somebody help me prove n! > 2n^3 where n>=7 by induction
OK. I.e., you need to show that , i.e., for .2(k^3)(k+1) > 2(k+1)^3
As a general idea, in a polynomial, a higher degree trumps all lower ones, regardless of coefficients, when is large. So, eventually will dominate . A usual way to show this is to find an upper bound of and to show that will dominate even this upper bound. To obtain an upper bound, note again that will dominate and 1. So replace by and 1 by : . (You need to show that this is true for .)
Where did we get so far? We can show that : indeed, for . In turn, we showed that for . Therefore, for , which is what we need.