1又1/2+2又1/6+3又1/12+4又1/20……+20又1/420
=(1+2+3+……+20）+（1/2+1/6+1/12+……+1/420)
=(1+20)×20÷2+(1-1/2+1/2-1/3+……+1/20-1/21)
=210-(1-1/21)
=210-20/21
=209又1/21