2/1*3+3/3*6+4/6*10+.+10/45*55
=(1-1/3)+(1/3-1/6)+(1/6-1/10)+...+(1/45-1/55)
=1-1/3+1/3-1/6+1/6-1/10+.+1/45-1/55
=1-1/55
=54/55