Solve |x²-3|<3 . This looks simple but i could not get the right answer. The answer is (-√5,-1)U(1,√5). How to solve this inequality?

1 Answer
Jul 15, 2015

The solution is that the inequality should be abs(x^2-3) < color(red)(2)x23<2

Explanation:

As usual with absolute values, split into cases:

Case 1: x^2 - 3 < 0x23<0

If x^2 - 3 < 0x23<0 then abs(x^2-3) = -(x^2-3) = -x^2+3x23=(x23)=x2+3
and our (corrected) inequality becomes:

-x^2+3 < 2x2+3<2

Add x^2-2x22 to both sides to get 1 < x^21<x2

So x in (-oo,-1) uu (1, oo)x(,1)(1,)

From the condition of the case we have

x^2 < 3x2<3, so x in (-sqrt(3), sqrt(3))x(3,3)

Hence:

x in (-sqrt(3), sqrt(3)) nn ((-oo,-1) uu (1, oo))x(3,3)((,1)(1,))

= (-sqrt(3), -1) uu (1, sqrt(3))=(3,1)(1,3)

Case 2: x^2 - 3 >= 0x230

If x^2 - 3 >= 0x230 then abs(x^2-3) = x^2+3x23=x2+3 and our (corrected) inequality becomes:

x^2-3 < 2x23<2

Add 33 to both sides to get:

x^2 < 5x2<5, so x in (-sqrt(5), sqrt(5))x(5,5)

From the condition of the case we have

x^2 >= 3x23, so x in (-oo, -sqrt(3)] uu [sqrt(3), oo)x(,3][3,)

Hence:

x in ((-oo, -sqrt(3)] uu [sqrt(3), oo)) nn (-sqrt(5), sqrt(5))x((,3][3,))(5,5)

= (-sqrt(5), -sqrt(3)] uu [sqrt(3), sqrt(5))=(5,3][3,5)

Combined:

Putting case 1 and case 2 together we get:

x in (-sqrt(5), -sqrt(3)] uu (-sqrt(3), -1) uu (1, sqrt(3)) uu [sqrt(3), sqrt(5))x(5,3](3,1)(1,3)[3,5)

=(-sqrt(5), -1) uu (1, sqrt(5))=(5,1)(1,5)