Bài $7$

`⇒A=-1/4`

`→A=a×1/3+a×1/4-a×1/6,a=-3/5`

`→A=(1/3+1/4-1/6)×-3/5`

`→A=5/12×-3/5`

`→A=-1/4`

`⇒B=1`

`→B=b×5/6+b×3/4-b×1/2,b=12/13`

`→B=(5/6+3/4-1/2)×12/13`

`→B=13/12×` `12/13`

`→B=1`

$#nguyenhai121210$

Đáp án+Giải thích các bước giải:

Bài 7:

A=a.$\frac{1}{3}$ + a. $\frac{1}{4}$ - a. $\frac{1}{6}$ 

A=$\frac{-3}{5}$.$\frac{1}{3}$ + $\frac{-3}{5}$. $\frac{1}{4}$ - $\frac{-3}{5}$. $\frac{1}{6}$ 

A=$\frac{-3}{5}$.($\frac{1}{3}$ +$\frac{1}{4}$ - $\frac{1}{6}$)

A= $\frac{-3}{5}$.($\frac{1.4}{3.4}$ +$\frac{1.3}{4.3}$ - $\frac{1.2}{6.2}$)

A=$\frac{-3}{5}$.($\frac{4}{12}$ +$\frac{3}{12}$ - $\frac{2}{12}$)

A=$\frac{-3}{5}$.$\frac{5}{12}$

A=$\frac{-15}{60}$ =$\frac{-3}{12}$ =$\frac{-1}{4}$ 

B=b.$\frac{5}{6}$ + b. $\frac{3}{4}$ - b. $\frac{1}{2}$ 

B=$\frac{12}{13}$.$\frac{5}{6}$ + $\frac{12}{13}$. $\frac{3}{4}$ - $\frac{12}{13}$. $\frac{1}{2}$ 

B=$\frac{12}{13}$.($\frac{5}{6}$ +$\frac{3}{4}$ - $\frac{1}{2}$)

B= $\frac{12}{13}$.($\frac{5.2}{6.2}$ +$\frac{3.3}{4.3}$ - $\frac{1.6}{2.6}$)

B=$\frac{12}{13}$.($\frac{10}{12}$ +$\frac{9}{12}$ - $\frac{6}{12}$ )

B=$\frac{12}{13}$.$\frac{13}{12}$

B=$\frac{144}{169}$ = $\frac{144}{169}$

Bài 8:

A=$\frac{7}{10.11}$+ $\frac{7}{11.12}$ +$\frac{7}{12.13}$...+ $\frac{7}{69.70}$

A=7.($\frac{1}{10.11}$+ $\frac{1}{11.12}$ +$\frac{1}{12.13}$...+ $\frac{1}{69.70}$)

A=7.($\frac{1}{10}$- $\frac{1}{11}$ +$\frac{1}{12}$+...+ $\frac{1}{69}$+$\frac{1}{70}$ )

A=7.($\frac{1}{10}$- $\frac{1}{70}$ )

A=7.$\frac{3}{35}$=$\frac{3}{5}$ 

B=$\frac{1}{25.27}$+ $\frac{1}{27.29}$ +$\frac{1}{29.31}$...+ $\frac{1}{73.75}$

B=$\frac{7}{25}$- $\frac{1}{27}$ +$\frac{1}{29}$-$\frac{1}{31}$ ...+ $\frac{1}{73}$-$\frac{1}{75}$ 

B=$\frac{1}{25}$-$\frac{1}{75}$=$\frac{2}{75}$ 

F=$\frac{4}{2.4}$ + $\frac{4}{4.6}$ + $\frac{4}{4.6}$ + $\frac{4}{6.8}$ +...+$\frac{4}{2008.2010}$  

F=2.($\frac{2}{2.4}$ + $\frac{2}{4.6}$ + $\frac{2}{4.6}$ + $\frac{2}{6.8}$ +...+$\frac{2}{2008.2010}$)  

F=2.($\frac{1}{2}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{6}$ +...+$\frac{1}{2008}$ - $\frac{1}{2010}$ )

F=2.($\frac{1}{2}$ - $\frac{1}{2010}$ ) 

F=2.$\frac{502}{1005}$=$\frac{1004}{1005}$ 

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