5/28 + 5/70 + 5/130 + ... + 5/700
Đặt A= 5/28 + 5/70 + 5/130 + ... + 5/700
A = 5/4.7 + 5/7.10 + 5/10.13 + ... + 5/25.28
3/5A = 3/4.7 + 3/7.10 + 3/10.13 + ... + 3/25.28
3/5A = 1/4 - 1/7 + 1/7 - 1/10 +......+ 1/25 - 1/28
3/5A = 1/4 - 1/28
3/5A = 3/14
3/5A= 3/14
A= 3/14 : 3/5
A= 5/14
Vậy 5/28 + 5/70 + 5/130 + ... + 5/700= 5/14
CHÚC CẬU HỌC TỐT!!!