Hi dignissimosminima,Good instinct. The wavy curve method plugs right in once you've opened the mod, but there's one spot in the negative case where people slip. Let me walk both cases exactly the way KarishmaB set them up.
Case 1: x ≥ 0, so |x| = xThe inequality becomes
(x − 2)(x + 5) < 0.
Mark the roots
−5 and
2 on a number line. For a product of two linear factors with positive leading coefficient, the wavy curve is
+ on the far right, then alternates:
+ above
2,
− between
−5 and
2,
+ below
−5.
We want
< 0, so take the negative stretch:
−5 < x < 2. But this case only allows x ≥
0, so we keep
0 ≤ x < 2.
Case 2: x < 0, so |x| = −xNow it's
(−x − 2)(x + 5) < 0. Factor out the
−1 to make the wavy curve usable:
−(x + 2)(x + 5) < 0 → (x + 2)(x + 5) > 0This is the step to watch: multiplying by
−1 flips the sign, so you're now solving
> 0, not < 0. (This is exactly the flip KarishmaB flagged for NetOrb.)
Roots are
−5 and
−2. Wavy curve:
+ outside the roots,
− between them. Since we want
> 0, take the outside pieces:
x < −5 or x > −2. This case only allows x <
0, so we keep
x < −5 or −2 < x < 0.
CombineUnion of both cases:
x < −5 or −2 < x < 2. Every value here is less than
2 - answer
B.
Quick habit to lock in the flip: solve
−(x − 1)(x − 3) < 0. Multiply by
−1 first →
(x − 1)(x − 3) > 0 → x <
1 or x >
3. If you forget to flip, you'd wrongly grab
1 < x < 3. Always flip the inequality the moment you multiply by a negative.
Answer: Bdignissimosminima
How would the solution go if i were to open up the mod for two cases x>0 and X<0 and then employ the wavey curve method?