$x+\frac{1}{x}\leq - 2$ for $x<0$?

Question

I did a similar one here. So I guess that to prove this one, we just need:

$$x+\frac{1}{x}\geq 2$$

As $x>0$ implies that $x+\frac{1}{x}$ is positive, then if $x<0$ then $x+\frac{1}{x}$ is the sum of two negative numbers and hence, negative. Then we just need to multiply both sides by $-1$:

$$-x-\frac{1}{x}\leq -2$$

As $x<0$, we can write:

$$x+\frac{1}{x}\leq -2$$

I'm not sure if I messed up something.

Substitute $z = -x$. $\displaystyle -z - \frac1{z} \le -2$ , $-z < 0$ which becomes $\displaystyle z + \frac1{z} \ge 2$ , $z > 0$ — , Aug 18 '16 at 05:52

score 1 · Answer 1 · answered Aug 18 '16 at 05:37

1

I think you proceeded right . other way round you can explain it this way- let x<0 let t= -x where t>0 so t + 1/t > or = 2 so. -x + -1/x > or =2 hence x +1/x

answered Aug 18 '16 at 05:37

Pewpaled

73

score 1 · Answer 2 · answered Aug 18 '16 at 05:42

1

Your approach is nice. You can also derive it like you did $x+\frac{1}{x}\geq 2$, by assuming $x$ is negative:

$$ (x+1)^2 \ge 0$$

$$x^2 + 2x + 1 \ge 0$$

$$x^2 + 1 \ge -2x$$

$$x+\dfrac 1x \le -2$$

answered Aug 18 '16 at 05:42

Ovi

23,737

$x+\frac{1}{x}\leq - 2$ for $x<0$?

2 Answers2