Question

Explain how to easily decide whether x - 6 is a factor of the polynomial P(x) = x ^ 7 - 5x ^ 5 + 2x ^ 4 - x ^ 2 + 9 without performing long division .

Answer 1

$\text{According to the factor theorem,}\\\\x-a~ \text{is a factor of}~ f(x)~ \text{if}~ f(a) = 0, ~ \text{where}~ a \in\mathbb{R}\\\\\text{Given that,}\\ \\P(x) = x^7 -5x^5 +2x^4 -x^2 +9\\\\P(6) = 6^7 -5\cdot 6^7 +2 \cdot 6^4 -6^2 +9 = 243621 \neq 0\\\\\text{Since} ~ P(6)\neq 0~, ~ x -6 ~ \text{is not a factor of P(x).}$