Polynomial Squares Sum
Let $ a, $ $ b, $ $ c $ be positive real numbers such that both
\[x^4 + ax^3 + bx^2 + cx + 1\]and
\[x^4 + 2ax^3 + 2bx^2 + 2cx + 1\]are both squares of polynomials. Find $ a + b + c $.
- 1
- 2
- 3
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- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
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- $[$
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- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
If $ x^4 + ax^3 + bx^2 + cx + 1 $ is the square of a polynomial, then it must be quadratic. We can assume that the quadratic is monic. Then to get a term of $ ax^3 $ when we square it, the coefficient of $ x $ in the quadratic must be $ \frac{a}{2} $. Hence,
\[x^4 + ax^3 + bx^2 + cx + 1 = \left( x^2 + \frac{a}{2} \cdot x + t \right)^2.\]Expanding, we get
\[x^4 + ax^3 + bx^2 + cx + 1 = x^4 + ax^3 + \left( \frac{a^2}{4} + 2t \right) x^2 + atx + t^2.\]Matching coefficients, we get
\begin{align*}
\frac{a^2}{4} + 2t &= b, \\
at &= c, \\
t^2 &= 1.\end{align*}Similarly, if $ x^4 + 2ax^3 + 2bx^2 + 2cx + 1 $ is the square of a polynomial, then we can assume the polynomial is of the form $ x^2 + ax + u $. Hence,
\[x^4 + 2ax^3 + 2bx^2 + 2cx + 1 = (x^2 + ax + u)^2.\]Expanding, we get
\[x^4 + 2ax^3 + 2bx^2 + 2cx + 1 = x^4 + 2ax^3 + (a^2 + 2u) x^2 + 2aux + u^2.\]Matching coefficients, we get
\begin{align*}
a^2 + 2u &= 2b, \\
2au &= 2c, \\
u^2 &= 1.\end{align*}From the equations $ at = c $ and $ 2au = 2c, $ $ t = \frac{c}{a} = u $. Thus, we can write
\begin{align*}
\frac{a^2}{4} + 2t &= b, \\
a^2 + 2t &= 2b, \\
at &= c, \\
t^2 &= 1.\end{align*}Since $ t^2 = 1, $ either $ t = 1 $ or $ t = -1 $. If $ t = 1, $ then $ \frac{a^2}{4} + 2 = b $ and $ a^2 + 2 = 2b $. Substituting for $ b, $ we get
\[a^2 + 2 = \frac{a^2}{2} + 4.\]Then $ a^2 = 4, $ so $ a = 2 $. Then $ b = 3 $ and $ c = 2 $.
If $ t = -1, $ then $ \frac{a^2}{4} - 2 = b $ and $ a^2 - 2 = 2b $. Substituting for $ b, $ we get
\[a^2 - 2 = \frac{a^2}{2} - 4.\]Then $ a^2 = -4, $ which has no real solutions.
Therefore, $ a = 2, $ $ b = 3, $ and $ c = 2, $ so $ a + b + c = \boxed{7} $.