Absolute value problem 1 by Dhaval Furia

Algebra Level pending

x 2 x 6 = x + 2 |x^{2} - x - 6| = x + 2

Find the product of the distinct roots of the equation above.

8 -8 4 -4 24 -24 16 -16

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2 solutions

Chew-Seong Cheong
Jun 12, 2020

Note that x 2 x 6 = ( x + 2 ) ( x 3 ) |x^2 - x - 6| = |(x+2)(x-3)| .

For x < 2 x < - 2 :

( x + 2 ) ( x 3 ) = x + 2 ( x + 2 ) ( x 3 ) = x + 2 x 2 x 6 = x + 2 x 2 2 x 8 = 0 ( x + 2 ) ( x 4 ) = 0 \begin{aligned} |(x+2)(x-3)| & = x + 2 \\ (x+2)(x-3) & = x + 2 \\ x^2 - x - 6 & = x + 2 \\ x^2 - 2x - 8 & = 0 \\ (x+2)(x-4) & = 0 \end{aligned}

There is no solution.

For 2 x < 3 -2 \le x < 3 :

( x + 2 ) ( x 3 ) = x + 2 ( x + 2 ) ( 3 x ) = x + 2 x 2 + x + 6 = x + 2 x 2 = 4 x = ± 2 \begin{aligned} |(x+2)(x-3)| & = x + 2 \\ (x+2)(3-x) & = x + 2 \\ -x^2 + x + 6 & = x + 2 \\ x^2 & = 4 \\ \implies x & = \pm 2 \end{aligned}

For x 3 x \ge 3 :

( x + 2 ) ( x 3 ) = x + 2 ( x + 2 ) ( x 3 ) = x + 2 x 2 x 6 = x + 2 x 2 2 x 8 = 0 ( x + 2 ) ( x 4 ) = 0 x = 4 Since x 3 \begin{aligned} |(x+2)(x-3)| & = x + 2 \\ (x+2)(x-3) & = x + 2 \\ x^2 - x - 6 & = x + 2 \\ x^2 - 2x - 8 & = 0 \\ (x+2)(x-4) & = 0 \\ \implies x & = 4 & \small \blue{\text{Since }x \ge 3} \end{aligned}

Therefore the product of the roots is 2 2 4 = 16 -2\cdot 2 \cdot 4 = \boxed{-16} .

Missed the "-" sign with 16 :)

Dhaval Furia - 12 months ago

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Thanks, I have added it.

Chew-Seong Cheong - 12 months ago

There are two possibilities :

(I) x 3 x\geq 3 or x 2 x\leq -2\implies In this case, the equation is ( x + 2 ) ( x 3 ) = x + 2 x = 4 , 2 (x+2)(x-3)=x+2\implies x=4,-2

(II) 2 x 3 -2\leq x\leq 3\implies In this case the equation is ( x + 2 ) ( 3 x ) = x + 2 x = 2 , 2 (x+2)(3-x)=x+2\implies x=2,-2

Hence the product of the distinct values of x x is 4 × 2 × ( 2 ) = 16 4\times 2\times (-2)=\boxed {-16} .

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