# Dont understand how they did this derivative (1 Viewer)

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#### leonne

1. The problem statement, all variables and given/known data
Dont understand how they did this derivative

2. Relevant equations
k=(d^2 (I))/dr^2
were(I)(r)=(-gm/r)+(j^2)/2r^2

3. The attempt at a solution

so using the k =... they got [d/dr((GM/r^2)-((j^2)/r^3)
than they got (-2GM/r^3)+((3j^2)/r^4) as answer I know how they got this but don't see how they got the first part like why its GM/r^2 and not -GM/r and why ((j^2)/r^3) and not (j^2)/2r^2 was thinking maybe they multiply and r because its dr^2 but what happen to the 2 in (j^2)/2r^2
thxs

#### tiny-tim

Homework Helper
hi leonne! (you must start using the X2 icon just above the Reply box … your post is very difficult to read )
… than they got (-2GM/r^3)+((3j^2)/r^4) as answer I know how they got this but don't see how they got the first part like why its GM/r^2 and not -GM/r and why ((j^2)/r^3) and not (j^2)/2r^2 was thinking maybe they multiply and r because its dr^2 but what happen to the 2 in (j^2)/2r^2
i'm confused you do know that d2I/dr2 is d/dr of dI/dr ?

#### leonne

o ok I get it now just take the derivative 2 times ill use the x2 next time lol keep on forgetting thxs

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