1. $5+10+15+...+5n=\frac{5n(n+1)}{2}$

2. $\frac{4}{5}+\frac{4}{5^2}+\frac{4}{5^3}+...+\frac {4}{5^n}=1-\frac{1}{5^n}$

3. $2^n>n^2$, for $n \ge 5$, $n \varepsilon \mathbb{N}$

4. $5.6+5.6^2+2.6^3+....+5.6^n=6(6^n-1)$

i would appreciate any help so thanks in advance!