Question

Помогите, пожалуйста, 7 номер

Answer 1

$\frac{5ab}{a+5b} *( \frac{a}{5b} - \frac{5b}{a} )= \frac{5ab}{a+5b}* \frac{a*a-5b*5b}{5ab} = \\ \\ = \frac{5ab}{a+5b} * \frac{a^2-(5b)^2}{5ab} = \frac{5ab*(a-5b)(a+5b)}{(a+5b)*5ab} = \\ \\ = \frac{1*(a-5b)*1}{1*1} = a-5b \\ \\ \\ a=5 \sqrt{7} +9 ; b= \sqrt{7} -9 \\ \\ 5 \sqrt{7} +9 - 5*( \sqrt{7} -9) = 5 \sqrt{7} +9 - 5 \sqrt{7} -5*(-9)=9+45=54$