y=x√x+3√x
=x^(3/2)+3x^(1/2)
所以y'=3/2*x^(1/2)+3/2*x^(-1/2)
所以y''=3/4*x^(-1/2)-3/4*x^(-3/2)
=3/4*(1/√x-1/x√x)
=3(x-1)/(4x√x)