let a complete binary tree tree structure is
a(1'st node)
/ \
b(2nd node) c(3)
/ \
d(4) e(5)
if it is put in an array it can be like in the order as first root,then root's left child,next roots right child ..so on.So the array will be
[a ---index1(as index starts from 1 as mentioned)
,b, --2
c,--3
d,--4
e--5
].
Now index 2's child(i.2e b' child) are d(index position 4) and e (index position 5)
that means to access node 2's child we need to use index 2*2 and 2*2+1
this is applicable for other nodes.
So if we generalize to access node i's child we need to access nodes 2i and 2i+1