Đáp án:
Giải thích các bước giải:
17) A=1/3+1/3^2+1/3^4+...+1/3^100
=>3A=1+1/3+1/3^2+1/3^4+...+1/3^99
=>3A-A=(1+1/3^2+1/3^4+...+1/3^99)-(1/3+1/3^2+1/3^4+...+1/3^100)
=>2A=1-1/3^100
=>2A=(1-1/3^100):2
18) 1/2+1/2^2+1/2^3+1/3^4+...+1/2^99
Đặt A=1/2+1/2^2+1/2^3+1/3^4+...+1/2^99
=>2A=2*(1/2+1/2^2+1/2^3+1/3^4+...+1/2^99)
=>2A=1+1/2+1/2^2+1/2^3+1/3^4+...+1/2^98
=>2A-A=(1+1/2+1/2^2+1/2^3+1/3^4+...+1/2^98)-(1/2+1/2^2+1/2^3+1/3^4+...+1/2^99)
=>A=1-1/2^99
