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For any real number $\alpha$, the parabola $f_{\alpha}(x) = 2x^2 + \alpha x + 3\alpha$ passes through the common point $(a, b)$. What is the value of $a + b$?

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Could you please use LaTeX to make your question readable? It is not very clear whether you meant to write $\,f(x)\,$ or $\,f(\alpha x)\,$ , for example. you use the FAQ section to get directions on this. –  DonAntonio Nov 29 '12 at 13:08
    
Wow! Great guessing work, @Michael Albanese ! –  DonAntonio Nov 29 '12 at 13:16

2 Answers 2

up vote 2 down vote accepted

So for any $\,\alpha\in\Bbb R\,$ ,we have that

$$b=f_\alpha(a)=2a^2+a\alpha+3\alpha\Longrightarrow $$

Since this is true for any $\,\alpha\in\Bbb R\,$ , let us choose:

$$\;\;\;\;\;\;\;\;\;\;\;\;\;\begin{align*}(1)\;\;\;\alpha=0:& \,\,b=2a^2\\(2)\;\;\;\alpha=1:&\,\,b=2a^2+a+3\end{align*}$$

Comparing (1)-(2), we get

$$a+3=0\Longrightarrow a=-3\Longrightarrow b=2\cdot 3^2=18\Longrightarrow a+b=15$$

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Choose two values for $\alpha=0,1$ and set $f_0(x)=f_1(x)$ to get: $$ 2x^2=2x^2+x+3 \\ x+3=0 \rightarrow x=-3 \rightarrow y=2(-3)^2=18 $$ Then $x+y=-3+18=15$.

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Since I can only choose 1 answer I choose the other and upvoted you –  chndn Dec 4 '12 at 11:54
    
@chndn fair enough... –  draks ... Dec 4 '12 at 11:55

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