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let's say that I need to calculate the following expression:

$$ \frac{\partial\mathrm{log}(\mathrm{exp}(w_1 * x_1 + b_1) + \mathrm{exp}(w_2 * x_2 + b_2))}{\partial w_1} $$

How do I start? The rules that I know about derivation, exponential and logarithm are not very useful in this situation.

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This is just an application of the chain rule

$$ \frac{\partial\mathrm{log}(\mathrm{exp}(w_1 * x_1 + b_1) + \mathrm{exp}(w_2 * x_2 + b_2))}{\partial w_1} = \frac{\partial}{\partial w_1}(log(f(w_1)) = \frac{\frac{\partial f(w_1)}{\partial w_1}}{f(w_1)} $$

So your final answer is:

$$ \frac{x_1\mathrm{exp}(w_1x_1+b_1)}{\mathrm{exp}(w_1 * x_1 + b_1) + \mathrm{exp}(w_2 * x_2 + b_2))}$$

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  • $\begingroup$ in $log(f(w))$ do you actually mean $log(f(w_1))$ ? $\endgroup$
    – fstab
    Oct 5, 2014 at 16:28
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    $\begingroup$ @francesco - yes, my mistake, I'll fix that. $\endgroup$
    – Eweler
    Oct 5, 2014 at 16:29

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