y=5-x+√(3x-1)令t=√(3x-1)3x-1=t²x=(t²+1)/3y=5-(t²+1)/3+t=5-t²/3-1/3+t=-1/3(t²-3t)+14/3=-1/3(t-3/2)²+14/3+3/4=-1/3(t-3/2)²+65/12开口向下，当t=3/2时，y有最大值：ymax=65/12值域：(-∞,65/12]
