Math Is Fun Forum

Marisca

Member

Registered: 2011-04-22

Posts: 53

Need help with expanding...

Hi,
For f(x)=3x^2+x-2 find: {x:f(x)>0}

for this question I expanded and got;

(3x-2)(x+1)>0, and from that x>2/3 and x>-1, which turns out as incorrect.

Could someone tell me where I went wrong?

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Need help with expanding...

Hi;

It is x < -1

Unlike a quadratic equation where you can set both factors equal to 0 and solve. You can not do that in an inequality. In other words, (x+1)(3x-2)>0 can not be solved by  (x+1)>0 or (3x-2)>0 as you have done. This is because both factors can also be negative!

Always check the graph or use your knowledge about general parabolas to get the answer. If you use the graph below then it is easy to see what the answer is.

Here is a link that will explain it all.

http://www.mathsisfun.com/algebra/inequ … lving.html

Remember when you are all done finding the answers always check by plugging in to the original question.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Marisca

Member

Registered: 2011-04-22

Posts: 53

Re: Need help with expanding...

Thank you!

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Need help with expanding...

Okay, see you later and glad to help!

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.