唔,早上好,刚起一个!化简有x=y/(y-9)代入得x+y=y(y-8)/(y-9)=f(y)求导有f'(y)=(y-12)(y-6)/(y-9)^2又由x,y是正整数,知y>9,又f(y)在(9,12)单调减,在(12,+∞)单调增,所以f(y)在y=12处取极小值,所以x=4,y=12x+y最小值为...