The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

C4 Q



Stuck on ciii)

any help?
gg.png317.4KB
Reply 1
Avatar for sytner9
sytner9
I'm afraid, If you can't do that, then you're going to struggle. I can't be bothered to give you an answer because typing that all out would take me far more time than I would be willing to give up in order to help you. I am sorry but I am just telling the truth. Sometimes life is just a bit s***.
Reply 2
Avatar for Notnek
Notnek
21
Original post by YaYaD


Stuck on ciii)

any help?


θ=72o\theta = 72^o is a solution of 8x3−4x+1=08x^3-4x+1=0 where x=cos⁡θx=\cos \theta

i.e. x=cos⁡72ox=\cos 72^o is a solution of that cubic.

So solving that cubic will give you an exact value for cos72ocos 72^o.

Are you able to solve the cubic? If not, post your working/ideas,
Reply 3
Avatar for davros
davros
16
Original post by YaYaD


Stuck on ciii)

any help?


In addition to what notnek said, part c(i) tells you that 2x-1 is a factor of the cubic expression, so you can use polynomial long division to find the other 2 solutions to the equation cubic = 0 (they will be roots of a certain quadratic),

You now have 3 solutions to the cubic equation which are all possible values of cos 72. You have to use a bit of thought to eliminate the 2 unwanted solutions.
(edited 9 years ago)
Reply 4
Avatar for YaYaD
YaYaD
OP
Original post by davros
In addition to what notnek said, part c(i) tells you that 2x-1 is a factor of the cubic expression, so you can use polynomial long division to find the other 2 solutions to the equation cubic = 0 (they will be roots of a certain quadratic),

You now have 3 solutions to the cubic equation which are all possible values of cos 72. You have to use a bit of thought to eliminate the 2 unwanted solutions.


Original post by notnek
θ=72o\theta = 72^o is a solution of 8x3−4x+1=08x^3-4x+1=0 where x=cos⁡θx=\cos \theta

i.e. x=cos⁡72ox=\cos 72^o is a solution of that cubic.

So solving that cubic will give you an exact value for cos72ocos 72^o.

Are you able to solve the cubic? If not, post your working/ideas,


when i solve 4x^2+2x-1=0
i get 2 answers root 5 - 1 / 4 and -root5 - 1 / 4 but im only ask to show on of them is exact right?
(edited 9 years ago)
Reply 5
Avatar for YaYaD
YaYaD
OP
Original post by sytner9
I'm afraid, If you can't do that, then you're going to struggle. I can't be bothered to give you an answer because typing that all out would take me far more time than I would be willing to give up in order to help you. I am sorry but I am just telling the truth. Sometimes life is just a bit s***.


you sound like a fkin dick tbh, no good deeds will come your way if you live life like that :smile:
Reply 6
Avatar for davros
davros
16
Original post by YaYaD
when i solve 4x^2+2x-1=0
i get 2 answers root 5 - 1 / 4 and -root5 - 1 / 4 but im only ask to show on of them is exact right?


Well, is cos 72 greater than 0 or less than 0?

So which of your possible answers is the one you want? :smile:

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you