Question

Làm ơn giúp với mai rồi

Answer 1

Bài 1)

1/1.2+1/2.3+1/3.4+1/4.5+...+1/19.20

= 1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/19-1/20

= 1/1-1/20

= 20/20-1/20

= 19/20

Answer 2

Bài 2)

A= 1/12+1/20+1/30+1/42+...+1/380

A= 1/3.4+1/4.5+1/5.6+1/6.7+...+1/19.20

A= 1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+...+1/19-1/20

A= 1/3-1/20

A= 20/60-3/60

A= 17/60

Answer 3

Đề khối 9 có sai ko bạn?