=360*（1/2+1/6+1/12+1/20+1/30） =360*[(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)] =360*(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6) =360*(1-1/6) =360*5/6 =300