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A text book that I'm reading has $k < -5$ as the solution for $k+5 > 0$. What I want to know is how this can be - why is it not $k > -5$?

Edit: looks like the text book's made a mistake.

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Because they made a mistake? –  Hans Lundmark Dec 20 '11 at 14:42
    
Yeah, that's what I'm thinking as well. But I just need to make sure because it doesn't make logical sense. The book is amazon.co.uk/Mathematics-Higher-Level-Fabio-Cirrito/dp/… –  Fortisimo Dec 20 '11 at 14:45

1 Answer 1

To get this question into the answered list:

The textbook made a mistake. Starting from $$k+5>0$$ we can add $-5$ to both sides, producing $$k+5+(-5)>0+(-5).$$ Because $5+(-5)=0$ and $0+(-5)=-5$, we have $$k+0>-5.$$ Finally, because $k+0=k$, we have $$k>-5.$$

Thus, the correct solution is $k>-5$, not $k<-5$.

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Dear Zev: Thanks for all your editing work!!! –  Pierre-Yves Gaillard Dec 20 '11 at 16:17
    
No problem! I am happy to help and it is excellent LaTeX practice :) –  Zev Chonoles Dec 20 '11 at 16:34

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