Question

[Kim Nam Hun]

Tinh nhanh:

\(\frac{7}{1\cdot5}+\frac{7}{5\cdot9}+\frac{7}{9\cdot13}+\frac{7}{13\cdot17}+\frac{7}{17\cdot21}\)

Answer 1

[Nguyễn Anh Quân]

 = 7/4.(4/1.5 + 4/5.9 + 4/9.13 + 4/13.17 + 4/17.21)

 = 7/4.(1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21)

 = 7/4.(1-1/21)

 = 7/4.20/21 = 5/3

Tk mk nha

Answer 2

[Mai Anh Tuấn]

Đặt biểu thức bằng A

4/7A=1-1/5+1/5-1/9+...+1/17_1/21

4/7A=1-1/21

4/7A=20/21

A=35/21=5/3