Đặt A= 5/14 + 5/35 +5/65 + ....+ 5/350
⇒ 1/2A= 5/28 + 5/70 + 5/130 +...+ 5/700
⇒1/2A=5/4.7 + 5/7.10 +5/10.13 +....+5/25.28
⇒1/10A =1/4.7 + 1/7.10 + 1/10.13 +...+1/25.28
⇒3/10A= 3/4.7 + 3/7.10 + 3/10.13 +...+3/25.28
⇒3/10A = 1/4-1/7+1/7-1/10+1/10-1/13+..+1/25-1/28
⇒3/10A=1/4-1/28
⇒3/10A=3/14
⇒A=3/14 : 3/10
⇒A=5/7
Mik tự lm ạ, ko coppy. Chúc bn học tốt!