Question

[le thanh ha]

tinh nhanh

  A=1/110+1/90+1/72+1/86+....+1/2

             ghi ca loi giai nhe !

Answer 1

[Đỗ Lê Tú Linh]

A=1/1*2+...+1/10*11

=1-1/2+...+1/10-1/11

=(1-1/11)+(1/2-1/2)+...+(1/10-1/10)

=(11/11-1/11)+0+0+...+0=10/11