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What value of $x$ satisfy $x^{2/3} + x^{1/3} - 2 \leq 0$?

  1. $-8\leq x \leq 1$
  2. $-1\leq x \leq 8$
  3. $1< x <8$
  4. $1\leq x \leq 8$
  5. $-8\leq x \leq 8$
in Quantitative Aptitude edited by
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Given that, $x^{\frac{2}{3}}+x^{\frac{1}{3}}-2\leq0$

$\Rightarrow x^{\frac{1}{3}}  \cdot x^{\frac{1}{3}} + x^{\frac{1}{3}} – 2\leq 0  \quad \longrightarrow (1)$

Let $x^{\frac{1}{3}} = k$

Now, $k \cdot k + k - 2\leq0$

$\Rightarrow k^{2} + k - 2\leq0$

$\Rightarrow k^{2} + 2k-k-2\leq0$

$\Rightarrow k(k+2)-1(k+2)\leq0$

$\Rightarrow (k+2)(k-1)\leq0$

$\Rightarrow \boxed{\left(x^{\frac{1}{3}}+2\right)\left(x^{\frac{1}{3}}-1\right)\leq 0} \quad \longrightarrow (2)$

$\textbf{Case 1:}\;x^{\frac{1}{3}}+2\leq0;$  $x^{\frac{1}{3}}-1\geq0$

$\Rightarrow x^{\frac{1}{3}}\leq-2;$  $x^{\frac{1}{3}}\geq1$

$\Rightarrow \boxed{x\leq-8; x\geq1}$



For $x$ solution is not possible.

$\textbf{Case 2:} \;x^{\frac{1}{3}}+2\geq0;$  $x^{\frac{1}{3}}-1\leq0$

$\Rightarrow x^{\frac{1}{3}}\geq-2;$  $x^{\frac{1}{3}}\leq1$

$\Rightarrow \boxed{x\geq-8; x\leq1}$



$\Rightarrow \boxed{-8\leq x\leq1}$

Correct Answer $:\text{A}$

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