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# Quick question - Finding stationary points watch

1. So on the Jan 2012 paper for Question 7a: http://pmt.physicsandmathstutor.com/...20C3%20AQA.pdf

I worked out dy/dx to be (1/4)x^2*e^-(1/4)x + 2x*e^-(1/2)x = 0

What I originally done was divide through by x - why is this not allowed? So I think I basically lost an x term. And so do I have to basically remember that whenever Im finding stationary points I can't divide through by x? Does this apply normally as well?
2. (Original post by MrToodles4)
So on the Jan 2012 paper for Question 7a: http://pmt.physicsandmathstutor.com/...20C3%20AQA.pdf

I worked out dy/dx to be (1/4)x^2*e^-(1/4)x + 2x*e^-(1/2)x = 0
You've got two errors in that derivative.

What I originally done was divide through by x - why is this not allowed? So I think I basically lost an x term. And so do I have to basically remember that whenever Im finding stationary points I can't divide through by x? Does this apply normally as well?
If you're trying to solve an equation equal to 0, you can only divide through by a term you know to be non-zero.

e.g

You can divide by since this term can never be zero.

You cannot divide by x, since x=0 is a solution, and you'd lose that.
3. (Original post by ghostwalker)
You've got two errors in that derivative.

Sorry I typed in wrong.

If you're trying to solve an equation equal to 0, you can only divide through by a term you know to be non-zero.

e.g

You can divide by since this term can never be zero.

You cannot divide by x, since x=0 is a solution, and you'd lose that.
Alright, that makes a lot of sense. Thank you so much I really appreciate it

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