For any AP, the expression for the nth term is always a
We know that nth term of an AP is given by a + (n-1)d where a = first term, d = common difference, an = nth term.
Rearranging further a + (n-1)d we get,
an=dn+(a−d) which is a linear expression in ‘n’. ('a’ and ‘d’ are constants, and the highest power of the variable n is 1)