Question

Tính A = -4/20+-4/30+-4/42+-4/56+-4/72+-4/90

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Answer 1

A=-4/4.5+-4/5.6+-4/6.7+-4/7.8+-4/8.9+-4/9.10

-1/4A=1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10

-1/4A=1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10

-1/4A=1/4-1/10

-1/4A=3/20

A=-3/5

Answer 2

thanks

Answer 3

Ta có : A = -4/20+-4/30+-4/42+-4/56+-4/72+-4/90

                = -4/4.5+-4/5.6+-4/6.7 +-4/7.8+-4/8.9+-4/9.10

                 =-4/4-(-4/5)+(-4/5)-(-4/6)+......+(-4/9)-(-4/10)

                  =-4/4-(-4/10)

                 =-3/5

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