y=(x-1)(x-2)(x-3)……(x-2005) 求导
两边同时取自然对数得：
lny=ln(x-1)+ln(x-2)+ln(x-3)+……+ln(x-2005)
两边同时对x求导得：
y '/y=1/(x-1)+1/(x-2)+1/(x-3)+……+1/(x-2005)
所以y '=y[1/(x-1)+1/(x-2)+1/(x-3)+……+1/(x-2005)]
=(x-1)(x-2)(x-3)……(x-2005)[1/(x-1)+1/(x-2)+1/(x-3)+……+1/(x-2005)]
为什么需要 y的导数比上y?