1 + 1 + 1 … + 1
____ ____ ____ ____
1×2 2×3 3×4 8×9
1 + 1 + 1 … + 1
____ ____ ____ ____
1×2 2×3 3×4 8×9
$\frac{1}{1.2}$ + $\frac{1}{2.3}$ + $\frac{1}{3.4}$ + … + $\frac{1}{8.9}$
= (1 – $\frac{1}{2}$) + ($\frac{1}{2}$ – $\frac{1}{3}$) + ($\frac{1}{3}$ – $\frac{1}{4}$) + … + ($\frac{1}{8}$ – $\frac{1}{9}$)
= 1 – $\frac{1}{2}$ + $\frac{1}{2}$ – $\frac{1}{3}$ + $\frac{1}{3}$ – $\frac{1}{4}$ + … + $\frac{1}{8}$ – $\frac{1}{9}$
= 1 – $\frac{1}{9}$
= $\frac{8}{9}$
Chúc học tốt!!!
Đáp án:
`1/{1.2}+1/{2.3}+1/{3.4}+…+1/{8.9}`
`=1-1/2+1/2-1/3+1/3-1/4+…+1/8-1/9`
`=1-1/9`
`=8/9`
$#lam$