$\dfrac{x-1988}{15}+\dfrac{x-1969}{17}+\dfrac{x-1946}{19}+\dfrac{x-1919}{21}=10
\\\Leftrightarrow (\dfrac{x-1988}{15}-1)+(\dfrac{x-1969}{17}-2)+(\dfrac{x-1946}{19}-3)+(\dfrac{x-1919}{21}-4)=0
\\\Leftrightarrow \dfrac{x-2003}{15}+\dfrac{x-2003}{17}+\dfrac{x-2003}{19}+\dfrac{x-2003}{21}=0
\\\Leftrightarrow (x-2003)(\dfrac{1}{15}+\dfrac{1}{17}+\dfrac{1}{19}+\dfrac{1}{21})=0$
$\Leftrightarrow x-2003=0\Leftrightarrow x=2003$ (vì $\dfrac{1}{15}+\dfrac{1}{17}+\dfrac{1}{19}+\dfrac{1}{21}\neq 0$)
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