Answer:
You can't use the formula a+(n-1)d for exactly the reason that you give (d changes). The formula is ONLY for arithmetic sequences where d remains constant.
The sequence 4, 9, 16, 25, ... is not arithmetic, but 4, 9, 16, 25,... are perfect squares.
a1 = 4 = 22
a2 = 9 = 32
a3 = 16 = 42, etc
So, we could define the sequence as an = (n+1)2, for n = 1,2,3,...
