Помогите пожалуйста. Даю 100 баллов. Алгебра, 8 класс

4)
$\frac{2a^2-2}{a-3}(\frac{2}{a+1}-\frac{1}{a-1}) + 3a = \frac{2(a-1)(a+1)}{a-3}*\frac{2(a-1)-a-1}{(a-1)(a+1)} + 3a = \\\\ =\frac{2(a-1)(a+1)}{a-3}*\frac{2a-2-a-1}{(a-1)(a+1)}+3a = \frac{2(a-1)(a+1)}{a-3}*\frac{a-3}{(a-1)(a+1)}+3a = 3a+2\\\\ \frac{1}{3} \ \textless \ a \ \textless \ \frac{1}{2}\\\\ 1 \ \textless \ 3a \ \textless \ 1.5\\\\ 3 \ \textless \ 3a+2 \ \textless \ 3.5\\$
Ответ: $(3;3.5)$

5)
$\frac{1}{t} = \frac{1}{a} + \frac{1}{b}\\\\ \frac{1}{t} = \frac{a + b}{ab}\\\\ t = \frac{ab}{a+b}\\\\$
$5 \leq a \leq 8\\ 20 \leq b \leq 24\\ 100 \leq ab \leq 192\\ 25 \leq a+b \leq 32\\\\$
$\frac{1}{32} \leq \frac{1}{a+b} \leq \frac{1}{25}\\\\ \frac{100}{32} \leq \frac{ab}{a+b} \leq \frac{192}{25}\\\\ 3\frac{1}{8} \leq \frac{ab}{a+b} \leq 7\frac{17}{25}\\\\ t \in [4;7]$