答：设两直角边长为x,20-x,则x和20-x都为正整数并且是质数：（1）当x=2,20-x=18,不满足条件；（2）当x=3,20-x=17,满足条件,面积S=x（20-x）/2=3*17/2=51/2；（3）当x=5,20-x=15,不满足条件；（4）当x=7,20-x=13,满足...