4/1*3 +4/3*5 +4/5*7 +……+4/19*21=2*(2/1*3+2/3/5+2/5*7+……+2/(19*21)=2[（3-1）/1*3+(5-3)/3*5+(7-5)*5*7+……+[(21-19)/19/21]=2*(1-1/3+1/3-1/5+1/5-1/7+……+1/19-1/21）=2*(1-1/21)=10/21