(1)解法一:x、y>0,由均值不等式得1/x+1/y=9 ==> 9>=2根号[1/(xy)] ==> xy>=4/81,故x+y>=2根号(xy)>=2×(2/9),即x+y最小值为4/9.(2)解法二:设x+y=t,代入题中等式整理得9x^2-9tx+t=0,其判别式不小于0,故81t^2-36t>=0 =...