Math HL help! quadratics:

[timtamboy63]

Hey guys, i've been working on this problem for an hour, i simply cannot get it!. Help would be greatly appreciated:

The roots A and B of the quadratic equation:

x^2-kx+(k+1)=0

are such that A^2+B^2 = 13

Find the possible calues of the real number k.

I've got these three equations, but i can't seemto get anywhere withthem:

A^2+B^2=13

A+B = k (b/c of the addition of roots rule)

AB = k+1 (b/c of the multiplication of roots rule)

Cheers to anyone that helps

[xbookgirlx]

Well if u have A+B=k and you square both sides to get A^2+B^2+2AB=k^2 and you plug in that AB=K+1 and then u get the equation A^2+B^2=K^2-2K-2

but we also know that A^2+B^2=13 and then if you solve that quadratic equation (0=K^2-2K-15) you should get the possible values for K and then you can find the roots of the original quadratic X^2+KX+(K+1)=0

[Bishup]

I followed the steps logically. And I get that K=(4+/-(136)^1/2)/2

That doesn't give me nice numbers though. Shame the 136 I got isn't 169.

[xbookgirlx]

i got k=5 or -3

[Bishup]

You are probs right then but I have never heard of those formulas/rules that the guy put up there.