tính nhanh 1/1+2+1/1+2+3+1/1+2+3+4+…+1/1+2+3+4+…..+50

1 bình luận về “tính nhanh 1/1+2+1/1+2+3+1/1+2+3+4+…+1/1+2+3+4+…..+50”

1. 1/1+2 + 1/1+2+3 +1/1+2+3+4 +…+1/1+2+3+…+50

   Ta có 2/2(1+2)+2/2(1+2+3)+…+2/2(1+2+…+50)

   =2/6+2/12+2/20+…+2/2550

   =2/2.3+2/3.4+…+2/50.51

   =2(1/2.3+1/3.4+…+1/50.51)

   =2(1/1-1/2+1/2-…+1/50-1/51)

   =2.(1-1/51)

   =2.50/51

   =100/51

   Trả lời