find the nth term of the 2,4,9,16,25
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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 2,4,9,16,25, ... is not arithmetic, but 2,4,9,16,... are perfect squares.
a1 = 2 = √2²
a2 = 4 = 2²
a3 = 9 = 3², etc
So, we could define the sequence as an = (n+1)², for n = 1,2,3,...