Question

найдите значение выражения...​

Answer 1

$( \frac{a + 3b}{ {a}^{2} - 3ab } - \frac{1}{a} ) \div \frac{b}{3b - a} = \\ = (\frac{a + 3b}{a(a - 3b)} - \frac{1}{a} ) \times \frac{3b - a}{b} = \\ = \frac{a + 3b - (a - 3b)}{a(a - 3b)} \times \frac{ - (a - 3b)}{b} = \\ = \frac{6b}{a} \times \frac{ - 1}{b} = - \frac{6}{a}$
При а=7,5 получаем:
$- \frac{6}{a} = - \frac{6}{7.5} = - \frac{60}{75} = \\ = - \frac{4}{5} = - 0.8$