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## Homework Statement

If vector r(t) is not 0, show that d/dt |r(t)| = (1/|r(t)|) r(t)dot r'(t).

## Homework Equations

The hint given was that |r(t)|^2 = r(t) dot r(t)

## The Attempt at a Solution

the post I saw about this question said that you should take the derivatives of both sides of the hint equation using chain rule, so I got:

2|r(t)|(1)=r'(t) dot r(t) + r(t) dot r'(t)

which is equal to:

|r(t)|= r'(t) dot r(t)

aaaaand now I don't know what to do

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