Hey,
Was the formula for the 2nd subtask also available in google search? I hope not.
Every problem has a pre-requisite. If you know the concept of inversion, then you can solve this problem from scratch without using google.
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Could you please provide a link to image or any thing related to how you found out the solution to the recurrence. I am stuck at some stepsā¦or any strategy that i should try.
By āformula for 2nd subtaskā you mean āsum of arithmetic progressionā? 
No.
In the first approach, the closed form of the nth term of recurrence. I find it harder than just a simple AP :P.
If you mean in inversion, then, yes, it is nāth term of an AP, but I feel it is very tricky to choose the correct origin.