1×3分之4—3×5分之8+5×7分之12—7×9分之16+……—19×21分之40等于多少

原式= (3+1)/(1×3)-(5+3)/(3×5)+(7+5)/(5×7)-(9+7)/(7×9)+……-(21+19)/(19×21)= (1+1/3)-(1/3+1/5)+(1/5+1/7)-(1/7+1/9)+……-(1/19+1/21)= 1+1/3-1/3-1/5+1/5+1/7-1/7-1/9+……-1/19-1/21= 1-1/21= 20/21...