Số số hạng của A là:(19-4):1+1=16(Số hạng)
Ta có A=(1/4+1/5+1/6+1/7)+(1/8+1/9+1/10+1/11)+(1/12+1/13+1/14+1/15)+(1/16+1/17+1/18+1/19)>(1/8+1/8+1/8+1/8)+(1/12+1/12+1/12+1/12)+(1/16+1/16+1/16+1/16)+(1/20+1/20+1/20+1/20)=1/8.4+1/12.4+1/16.4+1/20.4+1/2+1/3+1/4+1/5+5/6+1/6=1
Vậy A>1