Nice! Thanks 
Can anyone post a nice link to better understand the concept of inversion ? There arenāt many tutorials about it clubbed with the programming category. Is it important?
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.