∣ x 2 − x − 6 ∣ = x + 2
Find the product of the distinct roots of the equation above.
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Missed the "-" sign with 16 :)
There are two possibilities :
(I) x ≥ 3 or x ≤ − 2 ⟹ In this case, the equation is ( x + 2 ) ( x − 3 ) = x + 2 ⟹ x = 4 , − 2
(II) − 2 ≤ x ≤ 3 ⟹ In this case the equation is ( x + 2 ) ( 3 − x ) = x + 2 ⟹ x = 2 , − 2
Hence the product of the distinct values of x is 4 × 2 × ( − 2 ) = − 1 6 .
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Note that ∣ x 2 − x − 6 ∣ = ∣ ( x + 2 ) ( x − 3 ) ∣ .
For x < − 2 :
∣ ( x + 2 ) ( x − 3 ) ∣ ( x + 2 ) ( x − 3 ) x 2 − x − 6 x 2 − 2 x − 8 ( x + 2 ) ( x − 4 ) = x + 2 = x + 2 = x + 2 = 0 = 0
There is no solution.
For − 2 ≤ x < 3 :
∣ ( x + 2 ) ( x − 3 ) ∣ ( x + 2 ) ( 3 − x ) − x 2 + x + 6 x 2 ⟹ x = x + 2 = x + 2 = x + 2 = 4 = ± 2
For x ≥ 3 :
∣ ( x + 2 ) ( x − 3 ) ∣ ( x + 2 ) ( x − 3 ) x 2 − x − 6 x 2 − 2 x − 8 ( x + 2 ) ( x − 4 ) ⟹ x = x + 2 = x + 2 = x + 2 = 0 = 0 = 4 Since x ≥ 3
Therefore the product of the roots is − 2 ⋅ 2 ⋅ 4 = − 1 6 .