Hello,

it's , not

Let

Hence

and

The equation is now :

Distribute the factors :

Change the indices so that you have in all sums :

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Now, change the starting n's in the sums, so that they start at the same point (namely n=2)

Similarly, we have :

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So we have the new equation :

Group the terms :

Now you'll have to use the initial conditions to find a recursive relation for

Hope that helps