Срочно решить задание. найти значение

$\frac{68}{9-\sqrt{13}}+\frac{2}{\sqrt{15}+\sqrt{13}}+\frac{85}{\sqrt{15}+10}=\\ \frac{68(9+\sqrt{13})}{68}+\frac{2(\sqrt{15}-\sqrt{13})}{2}+\frac{85(\sqrt{15}-10)}{-85}=\\ 9+\sqrt{13}+\sqrt{15}-\sqrt{13}-\sqrt{15}+10=19\\$

2) $\frac{1}{4*6}+\frac{1}{6*8}+\frac{1}{8*10}+\frac{1}{10*12}+\frac{1}{12*14} + \frac{1}{14*16} + \frac{1}{16*18} + \frac{1}{18*20}=\\ \frac{1}{8*3}+\frac{1}{6*8}+\frac{1}{8*10}+\frac{1}{8*15}+\frac{1}{8*21}+\frac{1}{8*28}+\frac{1}{8*36}+\frac{1}{8*45}=\frac{1}{10}$
3)
$b+\frac{1}{b}=9\\ (b+\frac{1}{b})^2=81\\ b^2+2+\frac{1}{b^2}=81\\ b^2+\frac{1}{b^2}=79$