原式=（6+7+……+20)+[1/(6*7)+1/(7*8)+……+1/(20*21)]
=(6+20)*(20-6+1)/2+(1/6-1/7+1/7-1/8+……+1/20-1/21)
=195+(1/6-1/21)
=195+5/42
=195又5/42