曲线的切向量可以表示为s=a(x'(t),y'(t),z'(t))=a(2t,3t^2,(2/3)t^(-1/3))
(1,1,1)处的切向量为s0=a(2,3,2/3)
已知so与oz正向z=(0,0,1)夹角为钝角,所以s0*z=2a/3