toán 7

Q

quynhphamdq

1/ Tính nhanh:
1/3x4-1/4x5-....-1/9x10
Ta có :
[TEX]\frac{1}{3.4}-\frac{1}{4.5}-...-\frac{1}{9.10}[/TEX]
[TEX]=\frac{1}{3.4}-(\frac{1}{4.5}+...+\frac{1}{9.10})[/TEX]
[TEX]=\frac{1}{3.4}-(\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10})[/TEX]
[TEX]=\frac{1}{3.4}-(\frac{1}{4}-\frac{1}{10})[/TEX]
[TEX]=\frac{1}{12}-\frac{3}{20}=\frac{-1}{15}[/TEX]
 
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