Найдите значение выражения №425-430, СРОЧНО!

$\frac{2a+b}{ab}- \frac{2}{b}= \frac{2a+b-2a}{ab}= \frac{b}{ab}= \frac{1}{a}$
при а=2/3 1/(2/3)=3/2=1,5

$\frac{a-3b}{ab}- \frac{1}{b}= \frac{a-3b-a}{ab}=- \frac{3b}{ab}=- \frac{3}{a}$
при а=3/17 -3/(3/17)=-17

$\frac{5a-4b}{ab}- \frac{5}{b}= \frac{5a-4b-5a}{ab}=- \frac{4b}{ab}=- \frac{4}{a}$
при а=16/5 -4/(16/5)=-5/4=-1,25

$\frac{6a}{a^2-b^2}- \frac{6}{a+b}= \frac{6a}{(a-b)(a+b)}- \frac{6}{a+b}= \frac{6a-6(a-b)}{(a-b)(a+b)}= \frac{6a-6a+6b}{a^2-b^2} = \frac{6b}{a^2-b^2}$
при а=8, b=2
$\frac{6*2}{8^2-2^2}= \frac{12}{64-4}= \frac{12}{60}= \frac{1}{5}=0,2$

$\frac{45a}{25a^-16b^2}- \frac{9}{5a+4b}= \frac{45a}{(5a-4b)(5a+4b)}- \frac{9}{5a+4b}= \frac{45a-9(5a-4b)}{(5a-4b)(5a+4b)}=$
$= \frac{45a-45a+36b}{(5a-b)(5a+b)}= \frac{36b}{(5a-4b)(5a+4b)}$
при а=5, b=5
$\frac{36*5}{(5*5-4*5)(5*5+4*5)}= \frac{36*5}{5*45}= \frac{36}{45}= \frac{4}{5}=0,8$

$\frac{7a}{a^2-4b^2}- \frac{7}{a+2b}= \frac{7a}{(a-2b)(a+2b)}- \frac{7}{a+2b}= \frac{7a-7(a-2b)}{(a-2b)(a+2b)}= \frac{14b}{(a-2b)(a+2b)}$
при а=8, b=3
$\frac{14*3}{(8-2*3)(8+2*3)}= \frac{14*3}{2*14}= \frac{3}{2}=1,5$