Logging inequalities 1

Algebra Level 4

Solve the inequality: x 2 x 6 < 2 x 3 \sqrt{x^2-x-6}<2x-3

x ( , ) x \in (-\infty,\infty) x [ 3 , ) x \in [3,\infty) x [ 3 , ) x \in [-3,\infty) x [ 7 , ) x \in [7,\infty)

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1 solution

Rishabh Jain
Feb 16, 2016

Nice question ... My solution is a bit lengthy but I tried not to skip any step!! ;-)
First we have to make a set in which we have to solve for x:
( 1. ) (1.) Since square root is defined only for non negative quantities, we get x 2 x 6 0 x^2-x-6\geq 0 or x ( , 2 ] [ 3 , ) \color{#D61F06}{x\in (-\infty,-2] \cup [3,\infty)} .
( 2. ) (2.) RHS of the inequality must be a non negative since square root of any quantity cannot be less than a negative, so we get 2 x 3 0 2x-3\geq0 or x [ 3 2 , ) \color{#D61F06}{x\in[\dfrac{3}{2},\infty)} .
From ( 1. ) (1.) and ( 2. ) (2.) we have to find solutions for our inequality in [ 3 , ) \color{#0C6AC7}{[3,\infty)} .


Squaring both sides of the inequality(since both sides are non negative) to get: x 2 x 6 < 4 x 2 12 x + 9 x^2-x-6<4x^2-12x+9 3 x 2 11 x + 15 > 0 \Rightarrow 3x^2-11x+15>0 We note that discriminate of 3 x 2 11 x + 15 3x^2-11x+15 is negative and leading coefficient is positive and hence inequality is satisfied x R \forall x\in\mathbb{R} . But we have to be cautious here before claiming the answer since we have to look only in the set we have previously found. x [ 3 , ) \huge\boxed{\color{#007fff}{\therefore x \in [3,\infty)}}

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