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How can we derive the chi-squared probability density function (pdf) using the pdf of normal distribution?

I mean, I need to show that

$$f(x)=\frac{1}{2^{r/2}\Gamma(r/2)}x^{r/2-1}e^{-x/2} \>, \qquad x > 0\>.$$

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    Did you read this? http://en.wikipedia.org/wiki/Proofs_related_to_chi-squared_distribution2011-10-10
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    The way you wrote this makes no sense. Did you mean $f_Y(x)$ rather than $f(Y)$? And it makes no sense to say "where X is..." when the foregoing statement doesn't mention anything called X.2011-10-10
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    @Harald: Please do not deface your question, I have undone your edits. There is still value in the question, perhaps someone else with the same confusion will come along and be helped by this post.2011-10-10
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    I have tried to restate some things in the question. However, the OP is still encouraged to edit it further to clarify, as only they know what precisely they are trying to ask.2011-10-11

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