4-2.Quadratic Equations and Inequations
hard

The set of all real numbers $x$ for which ${x^2} - |x + 2| + x > 0,$ is

A

$( - \infty ,\,\, - 2)\, \cup (2,\,\infty )$

B

$( - \infty ,\,\, - \sqrt 2 )\, \cup (\sqrt 2 ,\,\infty )$

C

$( - \infty ,\,\, - 1)\, \cup (1,\,\infty )$

D

$(\sqrt 2 ,\,\infty )$

(IIT-2002)

Solution

(b) Case I: When $x + 2 \ge 0$ i.e. $x \ge – 2,$

Then given inequality becomes

${x^2} – (x + 2) + x > 0$ ==> ${x^2} – 2 > 0 \Rightarrow \,\,|x|\, > \sqrt 2 $

==> $x < – \sqrt 2 $ or $x > \sqrt 2 $

As $x \ge – 2,$ therefore, in this case the part of the solution set is $[ – 2, – \sqrt 2 ) \cup (\sqrt 2 ,\infty ).$

Case II: When $x + 2 \le 0$ i.e. $x \le – 2,$

Then given inequality becomes ${x^2} + (x + 2) + x > 0$

$ \Rightarrow {x^2} + 2x + 2 > 0$ $ \Rightarrow {(x + 1)^2} + 1 > 0,$ which is true for all real $x$

Hence, the part of the solution set in this case is $( – \infty , – 2]$.

Combining the two cases, the solution set is

$( – \infty , – 2) \cup ([ – 2, – \sqrt 2 ] \cup (\sqrt 2 ,\infty )$ $ = ( – \infty , – \sqrt 2 ) \cup (\sqrt 2 ,\infty ).$

Standard 11
Mathematics

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