Without expanding, show that a-b is a factor of |A| if
A=\left[\begin{array}{lll}1 & 1 & 1 \\a & b & c \\a^2 & b^2 & c^2\end{array}\right]Matrix A: [1 1 1; a b c; a^2 b^2 c^2]
=(a-b)\left|\begin{array}{ccc}0 & 1 & 1 \\ 1 & b & c \\ a+b & b^2 & c^2\end{array}\right| \quad \begin{aligned} & a-b \text { is a common factor } \\ & \text { of column 1 }\end{aligned}
Hence, |A| is equal to a-b times the last determinant, and so a-b is a factor of |A|.