# Spot the mistakes

1. Oct 2, 2008

### garyljc

1. The problem statement, all variables and given/known data
Spot the mistakes of a student

2. Relevant equations
1/x < x < 1
therefore 1<x^2
therefore 1<x
but x<1 therefore there are no solution

3. The attempt at a solution
the questions requires me to spot the mistake made by a student
so first of all
in the 2nd line , it reads 1<x^2 . this statement is wrong since the student assumes that x is always >0

i'm not too sure about about the second one , namely 1<x . Since it's related to the first one . What should i put ?
is there any other mistakes i have not spotted ?

2. Oct 2, 2008

### danago

If you are given the statement x2 > 1, it doesnt necessarily imply that x>1. Consider the case where x=-5. x2=25 which is certainly greater than 1, but x>1 is not true. Again, the student has assumed that x is positive.

3. Oct 2, 2008

### garyljc

so does it mean there's only one mistake ?

4. Oct 2, 2008

If $$x > 0$$ you can't have

$$0 < \frac 1 x < x < 1$$

because this is equivalent to

$$0 < 1 < x^2 < x$$

5. Oct 2, 2008

### logarithmic

If 1/x < x < 1, it does not necessarily follow that 1 < x^2.

For example, for x=-1/2, we have -2 < -1/2 < 1, but 1 < 1/4 is false.

In fact, the implication is false for all -1 < x < 0.

6. Oct 2, 2008

If the previous post was directed at mine, you missed one of my points.
I said
If $$x > 0$$ you can't have

$$0 < \frac 1 x < x < 1$$

because (if you multiply through the inequality by $$x$$) then you would have

$$0 < 1 < x^2 < x$$

My initial comment ruled out negative values from consideration. logarithmic, if I misunderstood you post by assuming it was meant at me, I apologize.