Divisible subaarays

#1

sir the array in the question was [1,3,2,6,4] which has a size of 5 elements
why then ,after you have created csum[] array which was[0,1,4,1,2,1] we add 0 at the beginning , so what is that 0 which makes the size of array as 6?
I have repeated the lecture many times and I cant get it

and second question sir, what is this formula Fc2?
what does it mean?

#2

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