View source for 23666: HW 4

'''Problem 16.''' Prove for all natural numbers $n\geq 5$:   $2^n>n^2$.

'''Problem 17.''' Let $n\in\mathbb{N}$. Conjecture a formula for the expression
\[a_n=\frac{1}{1\cdot 2}+\frac{1}{2\cdot 3}+\frac{1}{3\cdot 4}+\cdots +\frac{1}{n\cdot (n+1)}\] and prove your conjecture by induction.

'''Problem 18.''' Use Problem 15 and induction to show that ${\cal P}(A)$ has $2^n$ elements, when $A$ has $n$ elements.