The definition of  subscript operator operator in C, according to (C99, 220.127.116.11p2), is that
E1[E2] is identical to (*((E1)+(E2)))
Compilers use pointer arithmetic internally to access array elements. And because of the conversion rules that apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th element of E1 (counting from zero).
Therefore, a[b] is defined as :
a[b] == *(a + b)
So will evaluate to
a == *(a + 8)
Here, a is a pointer to the first element of the array and a is the value of an elements which is 8 elements further from a, which is the same as *(a + 8) and 8[a] will evaluate to following which means both are same.
8[a] == *(8 + a)
So by addition commutative property, a == 8[a]
Sample program showing above results :
a is 6 5[a] is 6
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