Question

a=1phan 15+1phan21+1phan28+1phan36+1phan45+1phan50+1phan60

Answer 1

A= 1/15 + 1/21+ 1/36 + 1/45 + 1/50 + 1/60

A= 211/  1050

nhé bạn

Answer 2

ta có 1/2a=1/30+1/42+1/56+1/72+1/90+1/110+1/132

              =1/5*6+1/6*7+1/7*8+1/8*9+1/9*10+1/10*11+1/11*12

              =1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10+1/10-1/11+1/11-1/12

              =1/5-1/12=7/60

suy ra a=7/30

Answer 3

mình nghĩ đề phải là a=1/15+1/21+1/28+1/36+1/45+1/55+1/66

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