siddhantvarma
Bunuel
Official Solution:
\(|x + 5|x|| < 12\)
What is the sum of all integers which satisfy the inequality above?
A. \(-2\)
B. \(-1\)
C. \(0\)
D. \(1\)
E. \(2\)
\(|x + 5|x|| < 12\)
\(-12 < x + 5|x| < 12\)
CASE 1: \(x < 0\).
\(-12 < x + 5(-x) < 12\)
\(-12 < -4x < 12\)
Multiply by \(-\frac{1}{4}\) (and don't forget to flip the signs since we multiply by a negative number): \(3 > x > -3\)
Negative integers in this range (don't forget that we consider \(x < 0\) for this case) are -1, and -2.
CASE 2: \(x \geq 0\).
\(-12 < x + 5x < 12\)
\(-12 < 6x < 12\)
Divide by \(6\): \(-2 < x < 2\)
Non-negative integers in this range (don't forget that we consider \(x \geq 0\) for this case) are 0, and 1.
The sum of all integers which satisfy the inequality is therefore: \(-1+(-2)+0+1=-2\)
Answer: A
|x+5|x||<12
Can be opened to equations:
x + 5x < 12 => 6x < 12 => x < 2
x - 5x < 12 => -4x < 12 => -x < 3 => x > -3
-x -5x < 12 => -6x < 12 => -x < 2 => x > -2
-x + 5x < 12 => 4x < 12 => x < 3
From the above 4 ranges of x we can say that -2 < x < 2 so integer values are -1, 0 and 1 which give the sum as 0.
I understand that putting x = -2 satisfies the equation, but where am I wrong in my approach?
Bunuel can you help please?
You cannot open modulus and not pay attention to the range.
When x > 0, then |x| = x, so we get:
|x + 5|x|| < 12
|x + 5x| < 12
6x < 12
x < 2
When x ≤ 0, then |x| = -x, so we get:
|x + 5|x|| < 12
|x - 5x| < 12
|-4x| < 12
Since x is negative or zero, then -4x is positive or 0, so we get:
-4x < 12
x > -3
Combining we get:
-3 < x < 2