<s>[INST] Derivative of capital Pi product I wanted to find the derivative of this function at $x=6$
$$y= \prod_{i=1}^{10} (x-i) = (x-1)(x-2) \cdots (x-10) $$
without expanding all of the brackets, so I used the product rule to find a pattern. However, the resulting sum tells me that the derivative is zero at every whole number which is obviously not true. I've been over my solution and I can't see how I've gone wrong. Please could someone highlight where I went wrong? Thank you in advance.
\begin{align*}
      \frac{\textit{d}y}{dx} &= (x-2)(x-3) \cdots (x-10) + (x-1) \frac{d}{dx} \biggl((x-2) \cdots (x-10) \biggr) \\
       &= \prod_{i=2}^{10} (x-i) +  (x-1) \frac{d}{dx} \biggl(\prod_{i=2}^{10} (x-i) \biggr) \\
       &= \prod_{i=2}^{10} (x-i) +  (x-1)\prod_{i=3}^{10} (x-i) + (x-1)(x-2)\frac{d}{dx} \biggl(\prod_{i=3}^{10} (x-i) \biggr) \\
                &= \prod_{i=2}^{10} (x-i) +  (x-1)\prod_{i=3}^{10} (x-i) + (x-1)(x-2)\biggl(\prod_{i=4}^{10} (x-i) \biggr) + \cdots \\
                      &= \frac{y}{x-1} + \frac{y}{x-2} + \frac{y}{x-3}+\cdots + \frac{y}{x-10} \\
                             &= \sum_{i=1}^{10} \biggl(\frac{y}{x-i}\biggr)
    \end{align*}
 [/INST] A good strategy for derivatives of products is the logarithmic derivative:
If $f(x)=\prod f_k(x)$ then $\ln f(x)=\sum \ln f_k(x)$ and by taking derivatives on both sides
$$\frac {f’}f =\sum \frac {f_k’}{f_k}$$
Strictly speaking, we need all $f_k(x)>0$, but using the product rule, you can show that $f(x)\ne 0$ is sufficient.
In your example, this takes you very far.
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