x1x + x2x + ... + x9x =
101*x + 10 + 101*x + 20 + ... + 101*x + 90 =
101*x + 101*x + ... + 101*x + 10 + 20 + ... + 90 =
909*x + 10(1+2+...+9) =
909*x + 10*45 =
909*x + 450
x5x = 101*x + 50
[tex]\frac{909\cdot x+450}{101\cdot x+50}= \\ \\ \frac{9(101\cdot x+50)}{101\cdot x+50}=9[/tex]