Tính nhanh các biểu thức sau

$\mathrm{D}=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}$$+\dots +\frac{1}{19.20}$
$\mathrm{A}=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}$$+\dots +\frac{1}{19.20}$
$\mathrm{c}=\frac{1}{9.10}-\frac{1}{8.9}-\frac{1}{7.8}$$-\dots -\frac{1}{2.3}-\frac{1}{2.1}$
$\mathrm{H}=\frac{1}{3}-\frac{3}{5}+\frac{5}{7}-\frac{7}{9}+\frac{9}{11}-\frac{11}{13}$$+\frac{13}{15}+\frac{11}{13}-\frac{9}{11}+\frac{7}{9}-\frac{5}{7}$$+\frac{3}{5}-\frac{1}{3}$
$\mathrm{k}=\frac{1}{7.11}+\frac{1}{11.15}+\frac{1}{15.19}+\dots +$$\frac{1}{2011.2015}$
$\mathrm{M}=\frac{1}{2013.2012}$$-\frac{1}{2012.2011}$$-\frac{1}{2011.2010}$$-...$$-\frac{1}{3.2}-\frac{1}{2.1}$