Question

Tính D=\(\frac{7}{10}+\frac{7}{10^2}+\frac{7}{10^3}+...\)                   (có vô hạn số hạng)

Answer 1

D=777777777777777777777777777777777777777777777.../1000000000000000000000000000000000000000000...

Answer 2

Gọi A=7/10+7/10^2+7/10^3+...7/10^n

10A=7+7/10+7/10^2+7/10^3+...+7/10^n-1

10A=7+(7/10+7/10^2+7/10^3+..+7/10n-1+7/10^n)-7/10^n

10A=7+A-7/10^n

9A=7-7/10^n

A=7.10^n-7/10^n/9

A=7.(10^n-1)/10^n/9