Year 12s: what will be the first way you research your future options? >>

C1 exam question, quadratics?

Uploaded with ImageShack.us

Hi, I've been having some problems with this question. But mainly with the last part.

I've got the first part, which is 4 +/- r5 and the second part.

But for the third part, after doing y^1/2 = 4+/-r5, and then doing y = (4+/-r5)^2. I'm not sure what to do. As what I did was (4+r5)(4-r5) and I got 11. :/

Reply 1

usycool1

You got the first part right I think, I'll just have a look at the second part.

Reply 2

MrGum

You need to do (4+sqrt5)^2 and (4-sqrt5)^2 - not a mixture of the two.

Reply 3

usycool1

Is the y to the power of 2/3 for question (iii)?

Reply 4

MedicalMayhem

Original post by Mr Gum

You need to do (4+sqrt5)^2 and (4-sqrt5)^2 - not a mixture of the two.

How'd you do that?

Original post by usycool1

Is the y to the power of 2/3 for question (iii)?

Sorry for the bad scan, but it's 1/2 :redface:

Reply 5

MrGum

(4+\sqrt 5)^2=4^2+4\sqrt5+4\sqrt5+(\sqrt 5)^2

similarly for

(4-\sqrt 5)^2

Reply 6

MedicalMayhem

Original post by Mr Gum

(4+\sqrt 5)^2=4^2+4\sqrt5+4\sqrt5+(\sqrt 5)^2

similarly for

(4-\sqrt 5)^2

oh, and then you add them together? :/

Reply 7

MrGum

No, there are two separate solutions for y (as hinted by the word "answers" in the question). It is a quadratic equation, so having two answers is not a problem.

Reply 8

MedicalMayhem

Original post by Mr Gum

No, there are two separate solutions for y (as hinted by the word "answers" in the question). It is a quadratic equation, so having two answers is not a problem.