Question

Tính tổng : 1/6 +1/12+1/20 +1/30+1/42+1/56+1/72+1/90​

​Giúp mik vs mik tick cko. Nhanh nka mik đag cần gấp.

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Answer 1

Gọi tổng đó là tổng S

Ta có: S = 1/6+1/12+1/30+1/42+1/56+1/72+1/90

=>      S = 1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8+1/8.9+1/9.10

=>      S = 1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10

=>      S = 1/2-1/10

=>      S = 5/10-1/10

=>      S=4/10

=>      S=2/5 

Answer 2

Phân số là \(\frac{2}{5}\)

Answer 3

ta thấy: 1/6=1/2x3; 1/12=1/3x4;1/20=1/4x5;.............;1/90=9x10

Đặt A= 1/6+1/12+1/20+...+1/90

A=1/2x3+1/3x4+1/4x5+.....+1/9x10

A=1/2 -1/3+1/3 -1/4+1/4 -1/5+.......+1/9 -1/10

A=1/2-(1/3+1/3)-(1/4+1/4)-......-(1/9+1/9)-1/10

A=1/2-0-0-0-.....0-1/10

A=1/2-1/10

A=2/5

Answer 4

kq bằng 9/10 mà