tính nhanh
a, \(\frac{3}{2\cdot5}+\frac{3}{5\cdot8}+\frac{3}{8\cdot11}+...+\frac{3}{17\cdot20}\)
b, \(\frac{5^2}{1\cdot6}+\frac{5^2}{6\cdot11}+\frac{5^2}{11\cdot16}+...+\frac{5^2}{26\cdot31}\)
c, \(\frac{1}{3}+\frac{1}{9}-\frac{1}{27}\) \(\frac{2}{3}-\frac{1}{4}+\frac{5}{11}\)
\(\frac{5}{3}+\frac{5}{9}-\frac{5}{27}\) \(\frac{5}{12}+1-\frac{7}{11}\)
a,=1/2-1/5+1/5-1/8+1/8-1/11+...+1/17-1/20
=1/2-1/20=19/20
b,=5.(1-1/6+1/6-1/11+...+1/26-1/31)
=5.(1-1/31)=5.30/31 =150/31