# Help understanding the chain rule to solve a differential equation

• Feb 8th 2013, 01:58 AM
matemauch
Help understanding the chain rule to solve a differential equation
I am following an example on a text book where the author is solving the next equation:

dp/dt = (p-900)/2

or if p is different than 900,

1/(p-900)dp/dt = 1/2 (so far so good for me)

But here the author states that by the chain rule the right hand side of the last equation is the derivative of ln|p-900| with respect to t:

d/dt (ln|p-900|) = 1/2

I really do not understand this last step. Any help will be very much appreciated :)
The reference of the book is: Boyce, DiPrima "Elementary Differential Equations"
• Feb 8th 2013, 02:13 AM
Prove It
Re: Help understanding the chain rule to solve a differential equation
If you were to differentiate \displaystyle \begin{align*} \ln{\left| p - 900 \right|} \end{align*} with respect to t, remembering that p is a function of t, we have

\displaystyle \begin{align*} \frac{d}{dt} \left( \ln{ \left| p - 900 \right| } \right) &= \frac{d}{dp} \left( \ln{ \left| p - 900 \right| } \right) \frac{dp}{dt} \\ &= \left( \frac{1}{p - 900} \right) \frac{dp}{dt} \end{align*}
• Feb 8th 2013, 02:27 AM
matemauch
Re: Help understanding the chain rule to solve a differential equation
Now it is clear for me. Thanks "Prove it"