求函数y=(x-1)(x-2)(x-3)…(x-100)的导数

求函数y=(x-1)(x-2)(x-3)…(x-100)的导数
已知lnx对x求导为1/x
lny=ln(x-1)+ln(x-2)+.+ln(x-100)
lny对x求导(lny)'先对中间变量y求导,y再对x求导
即为y'/y
ln(x-1)+ln(x-2)+.+ln(x-100)对x求导
和的导数等于导数的和
[ln(x-1)+ln(x-2)+.+ln(x-100)]'=[ln(x-1)]'+[ln(x-2)]'+...+[ln(x-100)]'
分别把x-1,x-2,...,x-100看成中间变量,
先对中间变量求导,中间变量再对x求导
[ln(x-1)]'+[ln(x-2)]'+...+[ln(x-100)]'=1/(x-1)+1/(x-2)+...+1/(x-100)
所以
y'/y=1/(x-1)+1/(x-2)+...+1/(x-100)
y'=y[1/(x-1)+1/(x-2)+...+1/(x-100)]
y'=(x-1)(x-2).(x-100)[1/(x-1)+1/(x-2)+...+1/(x-100)]
为什么 lny对x求导(lny)'先对中间变量y求导,y再对x求导
即为y'/y?(lny)'不是应该=1/y吗?为什么=y'/y?