My Problem Number 6

Algebra Level 4

3 ( x 2 ) + 4 2 x 3 3 x + 1 2 ( x 2 1 ) = 1 \dfrac{3(x-2) + 4\sqrt{2x^3-3x+1}}{2(x^2-1)} = 1

Find the minimum root of that satisfy the equation above.


The answer is -0.5.

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2 solutions

I know this way is a bit too tedious, but I'm posting this anyway.

For all x ± 1 x \ne \pm 1 , multiplying both sides by 2 ( x 2 1 ) 2(x^2-1) we get:

3 ( x 2 ) + 4 2 x 3 3 x + 1 = 2 ( x 2 1 ) 3(x-2)+4 \sqrt {2x^3-3x+1} = 2(x^2-1)

4 2 x 3 3 x + 1 = 2 x 2 3 x + 4 \Leftrightarrow 4 \sqrt {2x^3-3x+1} = 2x^2-3x+4

As 2 x 2 3 x + 4 > 0 2x^2-3x+4 > 0 for all real x x , we can square both sides:

16 ( 2 x 3 3 x + 1 ) = ( 2 x 2 3 x + 4 ) 2 16(2x^3-3x+1)=(2x^2-3x+4)^2

16 ( 2 x 3 3 x + 1 ) = 4 x 4 12 x 3 + 25 x 2 24 x + 16 \Leftrightarrow 16(2x^3-3x+1)=4x^4-12x^3+25x^2-24x+16

4 x 4 44 x 3 + 25 x 2 + 24 x = 0 \Leftrightarrow 4x^4-44x^3+25x^2+24x=0

x ( 2 x + 1 ) ( 2 x 2 23 x + 24 ) = 0 \Leftrightarrow x(2x+1)(2x^2-23x+24)=0

Two roots of 2 x 2 23 x + 24 2x^2-23x+24 are both positive because their sum is 23 2 > 0 \frac {23} {2} > 0 and their product is 12 > 0 12 > 0 Hence the minimum root is x = 1 2 x=\boxed {-\frac {1} {2}}

Nice and elegent! By the way, did u use a particular trick to factorise the quartic?

Yogesh Verma - 6 years, 3 months ago
Nut Nutthapong
Apr 27, 2015

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