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VCE Stuff => VCE Mathematics => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Mathematical Methods CAS => Topic started by: nerd on November 06, 2008, 03:53:31 pm

Title: Question time...
Post by: nerd on November 06, 2008, 03:53:31 pm: Question 3, MAV Exam 1 2007

I used the discriminant must be greater than 0, yet that is only part of the answer...

I don't really understand the solutions!

Thanks in advnace :P
Title: Re: Question time...
Post by: hamtarofreak on November 06, 2008, 04:57:44 pm: e^x is always > 0, so you must disregard all values that would imply e^x being < 0.

i.e. where $5 - \Delta < 0$

$5 - \sqrt{25+4k} < 0$

$5 < \sqrt{25+4k}\$

$25 < 25+4k\$

$0 < 4k$

Disregard these values.

k must be < 0 and > -25/4
$\frac{-25}{4} < k < 0$
Title: Re: Question time...
Post by: bucket on November 06, 2008, 05:21:50 pm: oh you legend! I just did that exam and was wondering the same thing :)
Title: Re: Question time...
Post by: onlyfknhuman on November 06, 2008, 05:59:49 pm: Quote from: hamtarofreak on November 06, 2008, 04:57:44 pm
e^x is always > 0, so you must disregard all values that would imply e^x being < 0.

i.e. where $5 - \Delta < 0$

$5 - \sqrt{25+4k} < 0$

$5 < \sqrt{25+4k}\$

$25 < 25+4k\$

$0 < 4k$

Disregard these values.

k must be < 0 and > -25/4
$\frac{-25}{4} < k < 0$

No idea whats going on...
Title: Re: Question time...
Post by: Mao on November 06, 2008, 06:10:47 pm: hamtarofreak: one small thing, $\Delta$ (the discriminant) does not include the square root, it is simply $b^2-4ac$

for a quadratic equation to have a solution, $\Delta \ge 0$ . and when it is negative it does not have any real solutions. :)