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Topic: Help Needed in math
Replies: 25   Last Post: May 2, 2014 6:01 PM

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Peter Percival

Posts: 1,215
Registered: 10/25/10
Re: Help Needed in math
Posted: Apr 29, 2014 8:03 AM
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William Elliot wrote:
> On Mon, 28 Apr 2014, Peter Percival wrote:
>> punisher wrote:
>>

>>> Find the largest values of the integer a for which ax^2-5x-3 is
>>> negative for all values of x.

>
>> Minus three.
>
> ax^2 - 5x - 3
> (5 +- sqr(25 + 12a)/2a
> 25 + 12a < 0; a < -25/12


At this point one concludes that -3 is the answer.

> ax^2 - 5x - 3 < -25x^2 / 12 - 5x - 3 < 0 ?
> -25x^2 - 60x - 36 = -(5x + 6)^2 <= 0
>



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