Tính K = 1/5 + 1/5^3 + 1/5^5 + ... + 1/5^99

Tính :
a) K = 1/5 + 1/5^3 + 1/5^5 +...+1/5^99
b) L = 1/3 - 1/3^2 + 1/3^3 - 1/3^4 +...+ 1/3^99 - 1/3^100
c) M = 1/5^2 - 1/5^4 + 1/5^6 - 1/5^8 +...+1/5^48 - 1/5^50