Bi-Quadratic

Calculus Level 4

Minimum value of ( x 1 ) ( x 2 ) ( x 3 ) ( x 4 ) (x-1)(x-2)(x-3)(x-4) .


The answer is -1.

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1 solution

Chew-Seong Cheong
Jan 24, 2016

( x 1 ) ( x 2 ) ( x 3 ) ( x 4 ) = ( x 1 ) ( x 4 ) ( x 2 ) ( x 3 ) = ( x 2 5 x + 4 ) ( x 2 5 x + 6 ) = ( x 2 5 x + 5 1 ) ( x 2 5 x + 5 + 1 ) = ( x 2 5 x + 5 ) 2 1 2 \begin{aligned} (x-1)(x-2)(x-3)(x-4) & = (x-1)(x-4)(x-2)(x-3) \\ & = (x^2-5x+4)(x^2-5x+6) \\ & = (x^2-5x+5-1)(x^2-5x+5+1) \\ & = \color{#3D99F6}{(x^2-5x+5)^2} -1^2 \end{aligned}

Since the term ( x 2 5 x + 5 ) 2 0 \color{#3D99F6}{(x^2-5x+5)^2} \ge 0 and ( x 2 5 x + 5 ) 2 = 0 \color{#3D99F6}{(x^2-5x+5)^2} = 0 when x = 5 ± 5 2 x = \dfrac{5 \pm \sqrt{5}}{2} , then the minimum value of ( x 1 ) ( x 2 ) ( x 3 ) ( x 4 ) = 0 1 = 1 (x-1)(x-2)(x-3)(x-4) = 0 - 1 = \boxed{-1} .

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