`[x-17]/1998+[x-21]/1994+[x+1]/1008=4`
`<=>[x-17]/1998-1+[x-21]/1994-1+[x+1]/1008-2=0`
`<=>[x-2015]/1998+[x-2015]/1994+[x-2015]/1008=0`
`<=>(x-2015)(1/1998+1/1994+1/1008)=0`
`=>x-2015=0`
`<=>x=2015`
\(\dfrac{x-17}{1998}+\dfrac{x-21}{1994}+\dfrac{x+1}{1008}\text{=}4\)
\(\Leftrightarrow\dfrac{x-17}{1998}+\dfrac{x-21}{1994}+\dfrac{x+1}{1008}-4\text{=}0\)
\(\Leftrightarrow\left(\dfrac{x-17}{1998}-1\right)+\left(\dfrac{x-21}{1994}-1\right)+\left(\dfrac{x+1}{1008}-2\right)\text{=}0\)
\(\Leftrightarrow\left(\dfrac{x-2015}{1998}\right)+\left(\dfrac{x-2015}{1994}\right)+\dfrac{x-2015}{1008}\text{=}0\)
\(\Leftrightarrow\left(x-2015\right)\left(\dfrac{1}{1998}+\dfrac{1}{1994}+\dfrac{1}{1008}\right)\text{=}0\)
\(\Leftrightarrow\left(x-2015\right)\text{=}0\)
\(\Leftrightarrow x\text{=}2015\)
\(vay...\)
