Modulus Inequalities (Absolute Value Inequalities): Problems with Solutions

Solution:
$|x^2+2x-4|>4 \Leftrightarrow $

$x^2+2x-4 > 4\\ x^2+2x-4 < -4$


$x^2+2x-4 > 4$
$x^2+2x-8 > 0$
$D=36$
$x_1= 2,\ \ x_2=-4$
$x^2+2x-4 > 4 \Leftrightarrow (x-2)(x+4)>0 \Leftrightarrow x\in(-\infin, -4)\cup(2, +\infin)$


$x^2+2x-4 < -4$
$x^2+2x < 0$
$x(x+2) < 0 \Leftrightarrow x\in(-2, 0)$


$x\in(-\infin, -4)\cup(2, +\infin)\\x\in(-2, 0) \\ \Leftrightarrow x\in (-\infin, -4)\cup (-2, 0) \cup (2, +\infin)$

Solution:
$x^2+|6x-24| \le 16$
$|6x-24| \le 16 - x^2$

$\begin{array}{|l} 6x-24 \le 16 - x^2 \\ 6x-24 \ge -(16 - x^2) \end{array}$

$\begin{array}{|l} 6x-24 - 16 + x^2\le 0 \\ 6x-24 \ge -16 + x^2 \end{array}$

$\begin{array}{|l} x^2 + 6x-40\le 0 \\ x^2 - 6x + 8 \le 0 \end{array}$

$x^2 + 6x-40\le 0$
$D=196$
$x_1=-10, \ \ \ \ x_2=4$
$x^2 + 6x-40\le 0 \Leftrightarrow (x-4)(x+10) \le 0 \Leftrightarrow x\in[-10,4]$


$x^2 - 6x + 8 \le 0$
$D=4$
$x_1=2, \ \ \ \ x_2=4$
$x^2 - 6x + 8 \le 0 \Leftrightarrow (x-2)(x-4) \le 0 \Leftrightarrow x\in[2,4]$


$\begin{array}{|l}x\in[-10,4] \\ x\in[2,4] \end{array} \Leftrightarrow x\in[2,4]$

Solution:

	$(-\infin,-3)$	$[-3,4)$	$(4,+\infin)$
$x+3$	-	+	+
$x-4$	-	-	+

Case 1. $x \in (-\infin,-3)$

$-(x+3)-(x-4)\le 11$
$-x-3-x+4\le 11$
$-2x+1\le 11$
$-2x\le 10$
$x\ge -5$ but $x\in(-\infin,-3) \Rightarrow x\in[-5,-3)$


Case 2. $x \in [-3,4)$

$(x+3)-(x-4)\le 11$
$x+3-x+4\le 11$
$0x\le 4\ \ \ \ - \forall x$ but $x \in [-3,4)$
$\Rightarrow x\in [-3,4)$


Case 3. $x \in (4,+\infin)$

$(x+3)+(x-4)\le 11$
$x+3+x-4\le 11$
$2x-1\le 11$
$2x\le 12$
$x\le 6$ but $x\in(4,+\infin) \Rightarrow x\in[4,6]$


Answer: $x\in[-5,-3) \cup [-3,4) \cup [4,6] \Leftrightarrow x\in[-5, 6]$

Solution:
$x^2-1=(x-1)(x+1)$

	$(-\infin,-1)$	$[-1,0)$	$[0, 1)$	$[1, +\infin)$
$(x-1)(x+1)$	+	-	-	+
$x$	-	-	+	+

Case 1. $x\in(-\infin, -1)$
$x^2 - 1 < x^2-(-x)+1$
$-1 < x+1$
$x>-2$, but $x\in(-\infin, -1) \Rightarrow x\in(-2, -1)$


Case 2. $x\in[-1, 0)$
$-(x^2 - 1) < x^2-(-x)+1$
$-x^2 + 1 < x^2+x+1$
$-2x^2-x<0$
$2x^2+x>0$
$x(2x+1)>0 \Leftrightarrow x \in (-\infin, -\frac12)\cup(0,+\infin)$
But $x\in[-1, 0) \Rightarrow x\in[-1, -\frac12)$


Case 3. $x\in[0,1)$
$-(x^2 - 1) < x^2-x+1$
$-x^2 + 1 < x^2-x+1$
$2x^2-x>0$
$x(2x-1)>0 \Leftrightarrow x \in (-\infin, 0)\cup(\frac12,+\infin)$
But $x\in[0, 1) \Rightarrow x\in[\frac12, 1)$


Case 4. $x\in[1,+\infin)$
$x^2 - 1 < x^2-x+1$
$x<2$
But $x\in[1,+\infin) \Rightarrow x\in[1, 2)$

Answer: $x\in(-2, -\frac12) \cup (\frac12, 2)$

Solution:
$x^2-2x-3 = 0$
$D=16, x_1=3, x_2=-1$
$x^2-2x-3 = (x-3)(x+1)$

	$(-\infin,-1)$	$[-1,2)$	$[2, 3)$	$[3, +\infin)$
$(x-3)(x+1)$	+	-	-	+
$x-2$	-	-	+	+

Case 1. $x\in (-\infin,-1)$
$x^2-2x-3+2(-(x-2))<5$
$x^2-2x-3-2(x-2)<5$
$x^2-2x-3-2x+4<5$
$x^2-4x-4<0$
$D=32,\ \ x_1=2-\sqrt{2},\ \ x_2=2+\sqrt{2}$
$\Rightarrow x^2-4x-4<0 \Leftrightarrow x\in (2-2\sqrt{2}, 2+2\sqrt{2})$
But $x\in (-\infin, -1)$
$\Rightarrow x\in \varnothing$


Case 2. $x\in [-1,2)$
$-(x^2-2x-3)+2(-(x-2))<5$
$-x^2+2x+3-2(x-2)<5$
$-x^2+2x+3-2x+4<5$
$-x^2+7<5$
$-x^2+2<0$
$x^2-2>0$
$(x-\sqrt{2})(x+\sqrt{2})>0 \Leftrightarrow x \in (-\infin, -\sqrt{2})\cup(\sqrt{2}, +\infin)$
But $x\in [-1,2) \Rightarrow x \in (\sqrt2, 2)$


Case 3. $x\in [2,3)$
$-(x^2-2x-3)+2(x-2)<5$
$-x^2+2x+3+2x-4<5$
$-x^2+4x-1<5$
$x^2-4x+6>0$
$D=-8 < 0 \Leftrightarrow \forall x$ is solution.
But $x\in [2,3) \Rightarrow x \in [2,3)$


Case 4. $x\in [3, +\infin)$
$(x^2-2x-3)+2(x-2)<5$
$x^2-2x-3+2x-4<5$
$x^2-12<0 \Leftrightarrow (x-2\sqrt3)(x+2\sqrt3) < 0$
$\Rightarrow x \in (-2\sqrt3, 2\sqrt3)$
But $x\in [3, +\infin) \Rightarrow x \in [3, 2\sqrt3)$


Answer: $x\in(\sqrt2, 2\sqrt3)$