1/2+1/6+1/12+1/20+1/30=1/2+1/(2*3)+1/(3*4)+1/(4*5)+1/(5*6)=1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6=1-1/6=5/6
原式 = 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 -1/5 + 1/5 -1/6 = 1 - 1/6 = 5/6
