应该是最大值吧y^2=2(1-x^2) [x√(1+y^2）]^2=x^2(1+y^2) =2x^2(3/2 - x^2) <=2[(x^2+(3/2-x^2))/2]^2=9/8, 当x^2=3/4时等号可以取到 所以x√(1+y^2）最大值为3√2/4
