1.Tính:
A=\(\frac{1}{5}\)+\(\frac{1}{5^2}\)+\(\frac{1}{5^3}\)+...+\(\frac{1}{5^{99}}\)
B=\(\frac{1}{1.2.3.4.5}\)+\(\frac{1}{2.3.4.5.6}\)+...+\(\frac{1}{n\left(n+1\right)\left(n+2\right)\left(n+3\right)\left(n+4\right)}\)
2.Cho A=2+22+23+...+210
Chứng minh: a,A\(⋮\)3
b,A\(⋮\)7
c,A\(⋮\)15
Help me, mình cần gấp T_T