Should we square it?

Algebra Level 5

x 2 x + 1 = x 3 + 2 x 2 3 x + 1 x 2 + 2 \large \sqrt{x^2-x+1}=\frac{x^3+2x^2-3x+1}{x^2+2}

Find the product of all real roots of the equation above.

Give your answer to 2 decimal places.


The answer is 0.97.

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1 solution

P C
Jul 9, 2016

Here's my solution. First, the constraint of x x is x > 3.0959... x>-3.0959... x 2 x + 1 = x 3 + 2 x 2 3 x + 1 x 2 + 2 \sqrt{x^2-x+1}=\frac{x^3+2x^2-3x+1}{x^2+2} ( x + 2 ) x 2 x + 1 + x 3 + 2 x 2 3 x + 1 x 2 + 2 ( x + 2 ) = 0 \Leftrightarrow (x+2)-\sqrt{x^2-x+1}+\frac{x^3+2x^2-3x+1}{x^2+2}-(x+2)=0 x 2 + 4 x + 2 x 2 + x 1 x + 2 + x 2 x + 1 + x 3 + 2 x 2 3 x + 1 x 3 2 x 2 2 x 4 x 2 + 2 = 0 \Leftrightarrow \frac{x^2+4x+2-x^2+x-1}{x+2+\sqrt{x^2-x+1}}+\frac{x^3+2x^2-3x+1-x^3-2x^2-2x-4}{x^2+2}=0 ( 5 x + 3 ) ( 1 x + 2 + x 2 x + 1 1 x 2 + 2 ) = 0 \Leftrightarrow (5x+3)\bigg(\frac{1}{x+2+\sqrt{x^2-x+1}}-\frac{1}{x^2+2}\bigg)=0 So we have the first root x = 3 5 x=\frac{-3}{5} . Now we consider 1 x + 2 + x 2 x + 1 1 x 2 + 2 = 0 \frac{1}{x+2+\sqrt{x^2-x+1}}-\frac{1}{x^2+2}=0 x 2 + 2 = x + 2 + x 2 x + 1 \Rightarrow x^2+2=x+2+\sqrt{x^2-x+1} x 2 x = x 2 x + 1 \Leftrightarrow x^2-x=\sqrt{x^2-x+1} ( x 2 x ) 2 = x 2 x + 1 \Leftrightarrow (x^2-x)^2=x^2-x+1 ( x 2 x ) 2 ( x 2 x ) 1 = 0 \Leftrightarrow (x^2-x)^2-(x^2-x)-1=0 We get x 2 x = 1 + 5 2 x^2-x=\frac{1+\sqrt{5}}{2} and x 2 x = 1 5 2 x^2-x=\frac{1-\sqrt{5}}{2} , solving these equation we get x = 1 ± 3 + 2 5 2 x=\frac{1\pm\sqrt{3+2\sqrt{5}}}{2} . Finally, the product is 0.97 \approx 0.97

Very nice algebraic massage, the key is to factor out 5x+3. I guessed on x = -3/5 after doing long division on RHS, but your method is much better.

Wei Chen - 4 years, 11 months ago

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