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find a non-constant polynomial function $p(x)$ such that $p(1)=p(2)=p(3)=4$. I try to solve it but do not know where to start (every time I substitute a number into the equation, I get three more unknown numbers). Would anyone give me any clue? Is there a general method to deal with this kind of problems?

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1 Answer 1

Hint: Any polynomial of shape $(x-1)(x-2)(x-3)q(x)$ will be $0$ at $x=1,2,3$.

Remark: Alternately, we can start as you did, and try to find a cubic $ax^3+bx^2+cx+d$ that will work. We get $3$ linear equations in $4$ unknowns, to which we can find a solution. Straightforward, but more calculation.

You asked for a general method. The idea of the hint above generalizes to Lagrange Interpolation. If you want to use the machinery of Lagrange Interpolation, to make sure it does not give you the trivial answer $p(x)=4$, you can specify, for example, that $p(0)=0$.

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i tried this was not correct –  raheem Nov 26 '13 at 19:28
    
The hint is a tiny step away from a full answer. Of course $(x-1)(x-2)(x-3)$ is not $4$ at $1$, $2$, or $3$. But a very small modification will fix that. I can write it down if you wish. –  André Nicolas Nov 26 '13 at 19:31
    
@raheem That's why it's labeled as a hint, not a full answer. –  Andreas Blass Nov 26 '13 at 20:10

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