$\frac{x-10}{1999}$ +$\frac{x-20}{1989}$ =$\frac{x-50}{1959}$ +$\frac{x-80}{1929}$
<=> ($\frac{x-10}{1999}$ -1)+ ($\frac{x-20}{1989}$-1) = ($\frac{x-50}{1959}$ -1) +($\frac{x-80}{1929}$ -1) <=> $\frac{x-2009}{1999}$ +$\frac{x-2009}{1989}$ =$\frac{x-2009}{1959}$ +$\frac{x-2009}{1929}$
<=> (x-2009)($\frac{1}{1999}$ +$\frac{1}{1989}$ -$\frac{1}{1959}$ -$\frac{1}{1929}$ )=0
<=> x -2009 =0 (do phần trong ngoặc lớn khác 0)
=> x = 2009
