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## Homework Statement

i am examining a proof of kepplers first law for my conics project and im a bit rusty on the physics, it begins withe the following line

## Homework Equations

To begin with, we will start off by applying Newton's law of motion and Newton's law of universal gravitation together to find that

m a = (-G m M/r^2) r

i think there is an extra r in this equation? if no where does it come from??

here is the link if it helps

http://members.kr.inter.net/joo/physics/curriculum/kepler/proof.html [Broken]

## The Attempt at a Solution

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