I'm so lost.

S

Smashed Ixnay

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Feb 22, 2006
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4
Find the exact values of the solution set for f(x) = 0, f (x) < 0 and f(x) > 0 for:

f(x) = 2x^2 - 5x - 12

nswer for = 0 is { -3/2, 4 }

I get { 5/4, -63/4 }

I used vertex formula there, but using quadratic formula I get the 5/4, but different answer for second part.
 
It appears you're just using the formula incorrectly.

Show us how you use the "vertex formula" to find where f(x) = 0.

You COULD just factor it. f(x) = (2x+3)(x-4)
 
tkhunny said:
It appears you're just using the formula incorrectly.

Show us how you use the "vertex formula" to find where f(x) = 0.

You COULD just factor it. f(x) = (2x+3)(x-4)
Click to expand...

For the vertex formula, I used -b/2(a). After I find my answer, I plug in every x in the equation to find the other point.

Now that you showed me that, I got the right answer. My only problem now is when to use vertex formula, quadratic formula, or a(x-h)^2+k formula. :?: Sometimes I can see when to use them, but like on this problem I would have no idea and I'd do it how I said (-b/2(a) and get the wrong answer, but it'd look right to me. Is there any tutorials around that explain this in depth?


Thank you for your help tkhunny! :)
 
Smashed Ixnay said:
tkhunny said:
It appears you're just using the formula incorrectly.

Show us how you use the "vertex formula" to find where f(x) = 0.

You COULD just factor it. f(x) = (2x+3)(x-4)
Click to expand...

For the vertex formula, I used -b/2(a). After I find my answer, I plug in every x in the equation to find the other point.

Now that you showed me that, I got the right answer. My only problem now is when to use vertex formula, quadratic formula, or a(x-h)^2+k formula. :?: Sometimes I can see when to use them, but like on this problem I would have no idea and I'd do it how I said (-b/2(a) and get the wrong answer, but it'd look right to me. Is there any tutorials around that explain this in depth?
Thank you for your help tkhunny! :)
Click to expand...

You might check out the purplemath.com pages (search for "quadratic"). They go into considerable detail and might make this clearer for you.
 
You might also NOT use a vertex formula when you are looking for intercepts. They would only coincidentally be the same.
 
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