Question

[Calone Alice (^-^)]

Tính tổng 

S=\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)

Answer 1

[Phan Linh]

S= 10/56+10/140+.....+10/1400

S= 5/28+5/70+.....+5/700

S= 5/4.7+5/7.10+......+5/25.28

S=5/3( 1/4-1/7+1/7-1/10+......+1/25-1/28)

S= 5/3.(1/4-1/28)

S= 5/3. 3/14

S= 15/42

VẬY S= 15/ 42