Ext maths problem need help asap pleaseeee (1 Viewer)

ebbygoo · Dec 15, 2013

Hey guys.

As per title, got a test tomorrow, and friend sent me a question asking how to do it. So I attempted it and got the same error as him. My working is here; I thought I was doing it right but get a negative under the square root.

The question is:
Provided you know two roots are reciprocals of each other of the polynomial; 3xcubed+ xsquared - kx + 6=0
Find k.

Spot my error? Haha

THAANKS IN ADVANCE!!

wagig · Dec 15, 2013

Your working's correct. One of the roots is complex.

sleepyair · Dec 15, 2013

I got beta=-2 as well, and for alpha I found it to be [-7-+sqrt(13)]/6.

Why is it so messy?

are you sure you typed up the question correctly? is the answer neat?

asianese · Dec 15, 2013

$\sum \alpha = \alpha+\frac{1}{\alpha} + \beta = -\frac{1}{3} \\ \sum\alpha\beta = \alpha\frac{1}{\alpha}+\alpha\beta + \frac{\beta}{\alpha} = \frac{k}{3} \\ 1+\beta\left(\alpha+\frac{1}{\alpha}\right) = \frac{k}{3} \\ \sum \alpha\beta\gamma = \alpha\frac{1}{\alpha}\beta = \beta = -6/3 = -2 \\ \alpha+\frac{1}{\alpha} = -\frac{1}{3} +2 = \frac{5}{3} \\ 1-2\cdot \frac{5}{3} = \frac{k}{3} \\ k = -7.$

The thing here to note is that you're not trying to find alpha - you are trying to find the value of k - so focus on this!

ebbygoo · Dec 15, 2013

Answer is 7, so asianese is spot on.

Thanks guys got it now.

ilyall5life

sleepyair · Dec 15, 2013

oh, silly me

thanks asianese

Ext maths problem need help asap pleaseeee (1 Viewer)

ebbygoo

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wagig

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sleepyair

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asianese

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ebbygoo

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sleepyair

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