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# Set theory watch

1. I get it up until x^2>9 x^2<16 , how can you just square this?

2. (Original post by khanpatel321)
I get it up until x^2>9 x^2<16 , how can you just square this?

Well, if x > 3 then x^2 > 9 - that should be clear from basic properties of positive numbers. If you know also that x < 4 then you must have x^2 < 16 too (you couldn't deduce that if you didn't have the x > 3 restriction because x might be a big negative number!)
3. (Original post by davros)
Well, if x > 3 then x^2 > 9 - that should be clear from basic properties of positive numbers. If you know also that x < 4 then you must have x^2 < 16 too (you couldn't deduce that if you didn't have the x > 3 restriction because x might be a big negative number!)
Ok I see, thank you.

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Updated: January 17, 2015
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