1/2+1/6+1/12+1/20+1/30+....+1/72+1/90= 1/1*2 +1/2*3 +1/3*4 +...+1/8*9 +1/9*10= (1/1 -1/2)+(1/2 -1/3)+...+(1/8 -1/9)+(1/9 -1/10)= 1/1 -1/10= 9/10
，，，
