Gọi 5/28 + 5/70 + 5/130 + ... + 5/700 là A
Ta có:
A= 5/28 + 5/70 + 5/130 + ... + 5/700
A= 5/4.7 + 5/7.10 + 5/10.13 + ... + 5/25.28
A x 3/5= 3/4.7 + 3/7.10 + 3/10.13 + ... + 3/25.28
A x 3/5= 1/4-1/7+1/7-1/10+1/10-1/13+...+1/25-1/28
A x 3/5= 1/4-1/28
A x 3/5= 3/14
A= 3/14 :3/5
A= 5/14
Chúc bn hok tốt!!!