a) $\dfrac{x+1}{65}$`+`$\dfrac{x+3}{63}$`=`$\dfrac{x+5}{61}$`+`$\dfrac{x+7}{59}$ 

$\dfrac{x+1}{65}$`+``1``+`$\dfrac{x+3}{63}$`+``1``=`$\dfrac{x+5}{61}$`+``1``+`$\dfrac{x+7}{59}$`+``1` 

$\dfrac{x+66}{65}$`+`$\dfrac{x+66}{63}$`=`$\dfrac{x+66}{61}$`+`$\dfrac{x+66}{59}$ 

$\dfrac{x+66}{65}$`+`$\dfrac{x+66}{63}$`-`$\dfrac{x+66}{61}$`-`$\dfrac{x+66}{59}$`=``0`

`(x+66).(`$\dfrac{1}{65}$`+`$\dfrac{1}{63}$`-`$\dfrac{1}{61}$`-`$\dfrac{1}{59}$`)``=``0` 

ĐK: $\dfrac{1}{65}$`+`$\dfrac{1}{63}$`-`$\dfrac{1}{61}$`-`$\dfrac{1}{59}$$\ne$`0`

`x+66=0`

`x=-66`

b) $\dfrac{x-6}{1998}$`+`$\dfrac{x-4}{2000}$`=`$\dfrac{x-2000}{4}$`+`$\dfrac{x-1998}{6}$ 

$\dfrac{x-6}{1998}$`-``1``+`$\dfrac{x-4}{2000}$`-``1``=`$\dfrac{x-2000}{4}$`-``1``+`$\dfrac{x-1998}{6}$`-``1`

$\dfrac{x-2004}{1998}$`+`$\dfrac{x-2004}{2000}$`=`$\dfrac{x-2004}{4}$`+`$\dfrac{x-2004}{6}$  

$\dfrac{x-2004}{1998}$`+`$\dfrac{x-2004}{2000}$`-`$\dfrac{x-2004}{4}$`-`$\dfrac{x-2004}{6}$`=``0`

`(x-2004).(`$\dfrac{1}{1998}$`+`$\dfrac{1}{2000}$`-`$\dfrac{1}{4}$`-`$\dfrac{1}{6}$`)``=``0`

ĐK: $\dfrac{1}{1998}$`+`$\dfrac{1}{2000}$`-`$\dfrac{1}{4}$`-`$\dfrac{1}{6}$$\ne$`0`

`x-2004=0`

`x=2004`

$#chithanh17062010$