If the set of all solutions of |x2+x-9|=|x|+|x2-9| is [ , ] [ , ), then ( 2+ 2+ 2 ) is equal to:
- 9
- 18
- 36
- 72
Answer
(2) 18
Solution
Write x2+x-9=x+(x2-9). So the equation is |a+b|=|a|+|b| with a=x, b=x2-9.
|a+b|=|a|+|b| ab 0
x(x2-9) 0 x(x-3)(x+3) 0
Sign scheme (critical points -3, 0, 3): the product is 0 for -3 x 0 and for x 3.
Solution set =[-3, 0] [3, )
=-3, =0, =3
2+ 2+ 2=9+0+9=18
Answer: (2)