Cho B = 1/4+1/5+1/6+...+1/19. Hãy chứng minh B < 1. B= 1/4+(1/5+1/6+...+1/9)+(1/10+1/11+...+1/19) Vì 1/5+1/6+...+1/9 > 1/9+1/9+...+1/9 nên 1/5+1/6+...+1/9 > 5/9 >1/2 Vì 1/10+1/11+...+1/19 > 1/19+1/19+...+1/19 nên 1/10+1/11+...+1/19 > 10/19 >1/2 ⇒ B > 1/4+1/2+1/2 > 1
