[tex]a = {10}^{2} - 10 - 9 = 100 - 10 - 9 = 81[/tex]
[tex]b = 2 \times (1 + 2 + 3 + ... + 9)[/tex]
[tex]b = 2 \times \frac{9(9 + 1)}{2} [/tex]
[tex]b = 9(9 + 1)[/tex]
[tex]b = 9 \times 10[/tex]
[tex]b = 90[/tex]
[tex] \frac{a}{b} = \frac{81}{90} = \frac{9}{10} = 0.9[/tex]
