令t=√x(根号x）;因为t=√x(根号x),且0≤x≤4,所以0≤t≤2y=√x(根号x）-x=t-t^2=-(t-1/2)^2+1/4;当(t-1/2)^2=0时,即t=1/2,x=t^2=1/4时,y最大,y的最大值是4;当(t-1/2)^2在0≤t≤2的范围内达到最大值时,即t=2,x=t^2=4...