Polynomial Division Sum
Let
\[x^8 + 98x^4 + 1 = p(x) q(x),\]where $ p(x) $ and $ q(x) $ are monic, non-constant polynomials with integer coefficients. Find $ p(1) + q(1) $.
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- -
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- $\frac{a}{b}$
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- 0
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- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
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- $\sin{}$
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- $\infty$
Solution
In order to factor the polynomial, we will try to solve the equation $ x^8 + 98x^4 + 1 = 0 $. First, we can divide both sides by $ x^4, $ to get $ x^4 + 98 + \frac{1}{x^4} = 0, $ so
\[x^4 + \frac{1}{x^4} = -98.\]Then
\[x^4 + 2 + \frac{1}{x^4} = -96,\]which we can write as $ \left( x^2 + \frac{1}{x^2} \right)^2 = -96 $. Hence,
\[x^2 + \frac{1}{x^2} = \pm 4i \sqrt{6}.\]Then
\[x^2 - 2 + \frac{1}{x^2} = -2 \pm 4i \sqrt{6},\]which we can write as
\[\left( x - \frac{1}{x} \right)^2 = -2 \pm 4i \sqrt{6}.\]To work with this equation, we will find the square roots of $ -2 \pm 4i \sqrt{6} $.
Assume that $ \sqrt{-2 + 4i \sqrt{6}} $ is of the form $ a + b $. Squaring, we get
\[-2 + 4i \sqrt{6} = a^2 + 2ab + b^2.\]We set $ a^2 + b^2 = -2 $ and $ 2ab = 4i \sqrt{6}, $ so $ ab = 2i \sqrt{6} $. Then $ a^2 b^2 = -24, $ so $ a^2 $ and $ b^2 $ are the roots of the quadratic
\[t^2 + 2t - 24 = 0,\]which factors as $ (t - 4)(t + 6) = 0 $. Hence, $ a^2 $ and $ b^2 $ are 4 and $ -6 $ in some order, which means $ a $ and $ b $ are $ \pm 2 $ and $ \pm i \sqrt{6} $ in some order.
We can check that
\[(2 + i \sqrt{6})^2 = 4 + 4i \sqrt{6} - 6 = -2 + 4i \sqrt{6}.\]Similarly,
\begin{align*}
(-2 - i \sqrt{6})^2 &= -2 + 4i \sqrt{6}, \\
(2 - i \sqrt{6})^2 &= -2 - 4i \sqrt{6}, \\
(-2 + i \sqrt{6})^2 &= -2 - 4i \sqrt{6}.\end{align*}Thus,
\[x - \frac{1}{x} = \pm 2 \pm i \sqrt{6}.\]If
\[x - \frac{1}{x} = 2 + i \sqrt{6},\]then
\[x - \frac{1}{x} - 2 = i \sqrt{6}.\]Squaring both sides, we get
\[x^2 - 4x + 2 + \frac{4}{x} + \frac{1}{x^2} = -6,\]so
\[x^2 - 4x + 8 + \frac{4}{x} + \frac{1}{x^2} = 0.\]This simplifies to $ x^4 - 4x^3 + 8x^2 + 4x + 1 $.
Similarly,
\[x - \frac{1}{x} = -2 + i \sqrt{6}\]leads to $ x^4 + 4x^3 + 8x^2 - 4x + 1 $. Thus,
\[x^8 + 98x^4 + 1 = (x^4 + 4x^3 + 8x^2 - 4x + 1)(x^4 - 4x^3 + 8x^2 + 4x + 1).\]Evaluating each factor at $ x = 1, $ the final answer is $ (1 + 4 + 8 - 4 + 1) + (1 - 4 + 8 + 4 + 1) = \boxed{20} $.