If A [1 .. n] is a max-heap, where may be the second, the third , the fourth... largest elements of the array?
[1]
[2] [3]
[4] [5] [6] [7]
As you know, the first element is the maximum. Following it are its children at positions 2*k and 2*k+1. So if you are 1 based, the next numbers in size are at 2 and 3.