Đặt S = 1 x 2 + 2 x 3 + ..... + 9 x 10

3S = 1 x 2 x (3-0) + 2 x 3 x (4 - 1) + ....... + 9 x 10 x (11 - 8)

3S = 1 x 2 x 3 + 2 x 3 x 4 - 1 x 2 x 3 + ......... + 9 x 10 x 11 - 8 x 9 x 10

3S = ( 1 x 2 x 3 -1 x 2 x 3) + ( 2 x 3 x4 - 2 x 3 x 4) + ..... + (8 x 9  x 10 - 8 x 9 x 10) + 9 x 10 x 11

3S = 9 x 10 x 11 = 990

S = 990 : 3 = 330     

A)   (-3/4 + 5/13):2/7-(2/1/2+8/13):2/7

\(=\)

\(=\)

B và C đâu bạn

a: \(=\left(-\dfrac{3}{4}+\dfrac{5}{13}\right)\cdot\dfrac{7}{2}-\left(\dfrac{5}{2}+\dfrac{8}{13}\right)\cdot\dfrac{7}{2}\)

\(=\dfrac{7}{2}\left(-\dfrac{3}{4}+\dfrac{5}{13}-\dfrac{5}{3}-\dfrac{8}{13}\right)\)

\(=\dfrac{7}{2}\cdot\dfrac{-413}{156}=\dfrac{-2891}{312}\)

b: \(=-\dfrac{3}{5}:\left(\dfrac{-2-5}{30}\right)+\dfrac{3}{5}:\left(-\dfrac{1}{3}-\dfrac{16}{15}\right)\)

\(=\dfrac{-3}{5}\cdot\dfrac{-30}{7}+\dfrac{3}{5}:\dfrac{-5-16}{15}\)

\(=\dfrac{3}{5}\cdot\dfrac{30}{7}+\dfrac{3}{5}\cdot\dfrac{-5}{7}\)

\(=\dfrac{3}{5}\cdot\dfrac{25}{7}=\dfrac{15}{7}\)

c: \(=3-\dfrac{1}{4}+\dfrac{2}{3}-5+\dfrac{1}{3}+\dfrac{6}{5}-6+\dfrac{7}{4}-\dfrac{3}{2}\)

\(=\left(3-5-6\right)+\left(\dfrac{-1}{4}+\dfrac{7}{4}-\dfrac{3}{2}\right)+\left(\dfrac{2}{3}+\dfrac{1}{3}\right)+\dfrac{6}{5}\)

\(=-8+1+\dfrac{6}{5}=-7+\dfrac{6}{5}=\dfrac{-35+6}{5}=\dfrac{-29}{5}\)