# I I can't understand the fact

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1. Jun 17, 2016

### funlord

I can's understand the fact about the equation

i cant prove the equation from the first attachment to the second attachment pls help.

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2. Jun 17, 2016

### andrewkirk

The second last formula is $\frac{dx}{dy}\frac d{dx}\left(\frac{dy}{dx}\right)^{-1}$.
If we write $y'$ for $\frac{dy}{dx}$ then that is
$$\frac1{y'}\frac d{dx}\left(\frac 1{y'}\right)$$
What happens when you perform the differentiation?

3. Jun 17, 2016

### funlord

Thank you very much, I can understand now why it has a negative sign