[tex]{3a + 5b = 20[/tex]
[tex]{5a + 3b = 8[/tex]
[tex] = > [/tex]
[tex]{3a + 5b = 20[/tex]
{a=8/5-3/5b
[tex] = > [/tex]
[tex]3( \frac{8}{5} - \frac{3}{5} b) + 5b = 20 = > b = \frac{19}{4} [/tex]
[tex] = > a = \frac{8}{5} - \frac{3}{5} \times \frac{19}{4} = > a = - \frac{5}{4 } = > (a.b) = ( - \frac{5}{4} . \frac{19}{4} )[/tex]
[tex]a)10a + 6b = 10 \times ( - \frac{5}{4} ) + 6 \times ( \frac{19}{4} ) = - 5 \times \frac{5}{2} + 3 \times \frac{19}{2} = - \frac{25}{2} + \frac{57}{2} = 16.[/tex]
[tex]b)15a + 25b = 15 \times ( - \frac{5}{4} ) + 25 \times \frac{19}{4} = - \frac{75}{4} + \frac{475}{4} = 100.[/tex]
[tex]c)a + b = - \frac{5}{9} + \frac{19}{4} = \frac{ - 5 + 19}{4} = \frac{14}{4} = \frac{7}{2} .[/tex]
[tex]d)a - b = - \frac{5}{4} - \frac{19}{4} = \frac{ - 5 - 19}{4} = \frac{ - 24}{4} = - \frac{24}{4} = - 6[/tex]
