Powers of Real Number

Algebra Level 2

Is there a real number x, such that it satisfies the following inequalities?

x^2 < x < x^3

No Yes

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2 solutions

Henry U
Dec 14, 2018

x 2 < x x < 1 x^2 < x \Rightarrow x < 1 , but this step can only be done for x > 0 x > 0 (otherwise the inequality sign has to be switched), so the condition is equivalent to 0 < x < 1 0 < x < 1 .

However, for these values, x 3 = x x 2 < x 2 x^3 = x \cdot x^2 < x^2 , so there are no solutions .


By the way, the possible orderings are

{ x 3 < x < x 2 , for < 1 x < x 3 < x 2 , for 1 < x < 0 x 3 < x 2 < x , for 0 < x < 1 x < x 2 < x 3 , for 1 < x \begin{cases} x^3 < x < x^2, \text{for } < -1 \\ x < x^3 < x^2, \text{for } -1 < x < 0 \\ x^3 < x^2 < x, \text{for } 0 < x < 1 \\ x < x^2 < x^3, \text{for } 1 < x \end{cases}

So, x 2 < x < x 3 x^2 < x < x^3 and x 2 < x 3 < x x^2 < x^3 < x are not possible.

Also, if x < 0 x < 0 then x 2 < x x > 1 x^2 < x \implies x > 1 is a contradiction.

Jordan Cahn - 2 years, 5 months ago
Tom Engelsman
Dec 16, 2018

LHS of this inequality: x 2 x < 0 x ( x 1 ) < 0 x ( 0 , 1 ) x^2 - x < 0 \Rightarrow x(x-1) < 0 \Rightarrow x \in (0,1) (i).

RHS of this inequality: x 3 x > 0 x ( x + 1 ) ( x 1 ) > 0 x ( 1 , 0 ) ( 1 , ) . x^3 - x > 0 \Rightarrow x(x+1)(x-1) > 0 \Rightarrow x \in (-1,0) \cup (1, \infty). (ii).

There are NO intervals of intersection between (i) and (ii), so no real solution x x exists.

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