Ta có:
S = $\frac{1}{2}$ + $\frac{1}{4}$ + $\frac{1}{8}$ + $\frac{1}{16}$
2 . S = 2 . ($\frac{1}{2}$ + $\frac{1}{4}$ + $\frac{1}{8}$ + $\frac{1}{16}$ + ...)
= 1 + $\frac{1}{2}$ + $\frac{1}{4}$ + $\frac{1}{8}$ + $\frac{1}{16}$ + ...
= 1 + ($\frac{1}{2}$ + $\frac{1}{4}$ + $\frac{1}{8}$ + $\frac{1}{16}$ + ...)
Ta thấy : $\frac{1}{2}$ + $\frac{1}{4}$ + $\frac{1}{8}$ + $\frac{1}{16}$ + ... = S
Suy ra :
2 . S = 1 + ($\frac{1}{2}$ + $\frac{1}{4}$ + $\frac{1}{8}$ + $\frac{1}{16}$ + ...)
= 1 + S
Vì 2 . S = 1 + S
=> 2 . S - S = 1
=> S = 1
=> ĐPCM
Chúc học tốt!!!