Square Root Conundrum


For all positive inter value of $x$, the value of $ 4x^4-8x^3+12x^2-8x+4 $ is a perfect square number. Verify mathematically.


Source: BdMO 2024 National Junior P2


Proof Based Problems  


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Solution

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$(2x^2-2x+2)^2 = 4x^4-8x^3+12x^3-8x+4$


Now, $2x^2-2x+2$ is always integer for integer $x$. So, $f(x)$ is always a perfect square.

Squaring a function with degree $n$ gives a function with degree $2n$ and the given is a quartic function. So, searching for a function with degree $2$ as $ax^2+bx+c$ is the best option.


$(ax^2+bx+c)^2 = a^x+2abx^3+(2ac+b^2)x^2+2bcx+c^2$. 


Comparing the coefficients, we get $a=2,c=2$ and $b=-2$

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