20.计算题
$\frac{8}{9}$×[$\frac{3}{4}$÷($\frac{7}{16}$-0.25)]
$\frac{3}{14}$×[7-4÷($\frac{2}{15}$+$\frac{2}{3}$)]
(2$\frac{2}{9}$+3$\frac{4}{7}$+1$\frac{4}{11}$)÷($\frac{3}{11}$+$\frac{5}{7}$+$\frac{4}{9}$)
1+3$\frac{1}{6}$+5$\frac{1}{12}$+7$\frac{1}{20}$+9$\frac{1}{30}$+11$\frac{1}{42}$
$\frac{\frac{1}{2}}{1+\frac{1}{2}}$+$\frac{\frac{1}{3}}{(1+\frac{1}{2})(1+\frac{1}{3})}$+$\frac{\frac{1}{4}}{(1+\frac{1}{2})(1+\frac{1}{3})(1+\frac{1}{4})}$+…+$\frac{\frac{1}{2013}}{(1+\frac{1}{2})(1+\frac{1}{3})(1+\frac{1}{4})+…+(1+\frac{1}{2013})}$
$\frac{1}{{2}^{2}-1}$+$\frac{1}{{4}^{2}-1}$+$\frac{1}{{6}^{2}-1}$+…+$\frac{1}{10{0}^{2}-1}$.