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Algebra Difficulty 2.2 Junior Find the answer

Which of the following is a possible value of xx if given two different numbers on a number line, the number to the right is greater than the number to the left, and the positions of x,x3x, x^{3} and x2x^{2} are marked on a number line?

A number or a short expression. Spacing and $ signs are ignored.

Solution

From the number line shown, we see that x<x3<x2x<x^{3}<x^{2}.
If x>1x>1, then successive powers of xx are increasing (that is, x<x2<x3x<x^{2}<x^{3} ).
Since this is not the case, then it is not true that x>1x>1.
If x=0x=0 or x=1x=1, then successive powers of xx are equal. This is not the case either.
If 0<x<10<x<1, then successive powers of xx are decreasing (that is, x3<x2<xx^{3}<x^{2}<x ). This is not the case either.
Therefore, it must be the case that x<0x<0.
If x<1x<-1, we would have x3<x<0<x2x^{3}<x<0<x^{2}. This is because when x<1x<-1, then xx is negative and we have x2>1x^{2}>1 which gives x3=x2×x<1×xx^{3}=x^{2} \times x<1 \times x. This is not the case here either.
Therefore, it must be the case that 1<x<0-1<x<0.
From the given possibilities, this means that 25-\frac{2}{5} is the only possible value of xx.
We can check that if x=25=0.4x=-\frac{2}{5}=-0.4, then x2=0.16x^{2}=0.16 and x3=0.064x^{3}=-0.064, and so we have x<x3<x2x<x^{3}<x^{2}. We can also check by substitution that none of the other possible answers gives the correct ordering of x,x2x, x^{2} and x3x^{3}.

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