[tex]a_n = (-1)^{n+1} n^2\\\\a_1 = (-1)^{1+1} \cdot 1^2 = (-1)^2 = 1\\\\a_2= (-1)^{2+1} \cdot 2^2 = (-1)^3 \cdot 4 = -4\\\\a_3 = (-1)^{3+1} \cdot 3^2 = (-1)^4 \cdot 9 = 9\\\\a_4 = (-1)^{4+1} \cdot 4^2 = (-1)^5 \cdot 16 = -16\\\\\text{The first four terms are:}~ 1, -4,~ 9, -16[/tex]
