instantaneous elasticity of substitute sigma=-{[u'(C(s)/u'(C(t))]/[C(s)/C(t)]} *{d[C(s)/C(t)]/d[u'(C(s)/u(C(t)]}, why taking the limit of that expression as s converges to t gives sigma=-=-u'(C(t)/u''C(t)*C(t), 请高手说明一下具体的数学过程，谢谢！！

公式写得太早糟糕了，是布兰查德 lecture 李第40页的。