For n = 1 LHS = 1 and RHS = 1 so n= 1 is true
Now Assume true for n = k k!\geq 2^{k-1}
Prove true for n = k+1, that is (k+1)!\geq 2^{k}
LHS = (k+1)! \\ = (k+1)(k)! \\ \geq (k+1)2^{k-1} \ \ \ \ \ \ \ (From assumption)\\ \geq 2.2^{k-1} \ \ \ \ \ \ \ \ \ (k+1\geq 2)\\ = 2^k
Therefore...