可以用 （u·v）=u'v+uv'f(x)=(3)'*x^(1/2)+3*[x^(1/2)]' =0+3*[(1/2)x^(-1/2)] =3/2*x^(1/2) =3√x /2x就是这样
f(x)=3√x=3*x^(1/2)f'(x)=3*[x^(1/2)]'=3*[(1/2)x^(-1/2)] = 3/2*1/√x
