已知0已知0
已知0x(9-x)=-x^2+9x=-(x^2-9x+81/4)+81/4=-(x-9/2)^2+81/4所以 x=9/2 时,有最大值 81/4
解,设y=x(9-x),则y=x(9-x)=-(x-9/2)^2+(9/2)^2即当x=9/2时,y最大且等于(9/2)^2