Question

найдите значение выражения
$frac{49}{7b- b^{2} } - frac{7}{b} b= -3$

Answer 1

$cfrac{49}{7b-b^2} - cfrac{7}{b}= cfrac{49-7(7-b)}{b(7-b)} = cfrac{49-49+7b}{b(7-b)} =cfrac{7b}{b(7-b)}=cfrac{7}{7-b}$

при b=-3   

$cfrac{7}{7-b} = cfrac{7}{7-(-3)}=cfrac{7}{7+3}=cfrac{7}{10}=0.7$