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I need to solve the following inequality and I have a couple of questions:

$\frac {1}{3} (x+5)>2x+10$

I get the process up untill $-x>5$ (assuming that's correct, please correct me if I'm wrong, lol!). I now need to multiply both sides by -1 to get a positive x.

According to a friend of mine, multiplying by -1 with inequalities causes the inequality sign (or whatever it's called) to switch. Meaning I'll get $x<-5$.

To me that just doesn't make any logical sense because I'm doing the exact same on each side of the ">"-sign, meaning I am not changing the relation between the two numbers? So why would the "greater than" become "less than"?

Is he correct? Is the answer $x<-5$? If so, what's the logic behind this?

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  • $\begingroup$ Please help me get the 1/3 fraction correct, lol. $\endgroup$ – Julian Nikolay Krogh-Fredrikse Oct 14 '15 at 0:32
  • $\begingroup$ Try \frac{1}{3} $\endgroup$ – Henricus V. Oct 14 '15 at 0:33
  • $\begingroup$ $\dots -3 < -2 < -1 < 0 < 1 < 2 < 3 \dots$ $\endgroup$ – BrianO Oct 14 '15 at 0:37
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He is correct. By multiplying $-1$ to both sides, you are mirroring the two numbers across $0$.(On the real line) Numbers on the "left" side of a number becomes the "right" side, and vise versa.

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  • $\begingroup$ Try "moving" each term to the other side of the inequality sign to make the x term positive... $\endgroup$ – DJohnM Oct 14 '15 at 5:55

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