Question

tính A=10/56 +10/140 + 10/260 +............ + 10/1400

Answer 1

bài nãy chia 2 ra rồi biến đổi mẫu 

=5/4.7+5/7.10+..................

sau đó tự làm nha

Answer 2

A=10/56+10/140+10/260+...+10/1400

A=5/28+5/70+5/130+...+5/700

A=5.(1/4.7+1/7.10+1/10.13+...+1/25.28)

A=5.1/3.(3/4.7+3/7.10+3/10.13+...+3/25.28)

A=5/3.(1/4-1/7+1/7-1/10+1/10-1/13+...+1/25-1/28)

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

A=5/3.3/14=5/14

Vậy A =5/14