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# Edexcel C3,C4 June 2013 Thread watch

1. (Original post by Better)
Can anyone help me please.

Question 7B C3 Solomon Press Paper B!

You have to verify why there is a root between 1 and 0. I took COS of both sides and then equated to 0 said this was f(x).

Then did f(0) , and f(1) - still got a negative and positive value which verified there was a root but I didn't get the same value for the the 0.

Why can't I take Cos of both sides, please anyone help, it's not many marks it's really bugging me. Surely algebraically it makes no difference?
How can you take cos of both sides?

Arccos(x-1)=√(x-2)yw
Cos(√x-2)=x-1
Cos(√x-2)-x+1= 0

I think usi ng arccos directly is not a good idea because arccos has a restricted domain.

Even though you get the same root for both methods their graph varies slightly close to 0 and 1.

f(x)= Cos(√x-2)-x+1
Or f(x)=arccos(x-1)-√x+2

These two functions are not equal so I think you can't expect to get the same values for 0 and 1. They only intersect when y=0.

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2. (Original post by MathsNerd1)
Yeah, I'm sure it was one of the last papers but I must admit that it caught me out for a moment, but only because I didn't have a clear definition of what irrational meant, but yeah, no one else could even attempt it at my 6th form, they just completely skipped it.

If they gave me anything like that in the exam I'll now be ready

On a side note, yesterday my teacher printed out the information that told me where I lost my marks and I already knew about 9 that I lost but then I apparently lost 2 more as I can't differentiate anymore, I'm seriously going to take my time in this attempt to make sure nothing like that would happen again.

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Yeah, once you have a clear definition of a rational number, the proof by contradiction is pretty easy to do
3. (Original post by justinawe)
Yeah, once you have a clear definition of a rational number, the proof by contradiction is pretty easy to do
Indeed, although I sometimes struggle with the actual method of proof by contradiction because I've never really been shown any examples, but it doesn't seem that hard really.

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4. (Original post by MathsNerd1)
Indeed, although I sometimes struggle with the actual method of proof by contradiction because I've never really been shown any examples, but it doesn't seem that hard really.

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Neither have I, but if you need examples you could always just google it
5. (Original post by StUdEnTIGCSE)
How can you take cos of both sides?

Arccos(x-1)=√(x-2)yw
Cos(√x-2)=x-1
Cos(√x-2)-x+1= 0

I think usi ng arccos directly is not a good idea because arccos has a restricted domain.

Even though you get the same root for both methods their graph varies slightly close to 0 and 1.

f(x)= Cos(√x-2)-x+1
Or f(x)=arccos(x-1)-√x+2

These two functions are not equal so I think you can't expect to get the same values for 0 and 1. They only intersect when y=0.

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Why are you not allowed to take cos of both sides.

f(x)= Cos(√x-2)-x+1

This is exactly what I put in my calculator thank you so much for checking this for me and your calculator looks incredible. How much did you get that for and will I be allowed to use it in the exam?
6. If tanxcos2x=2cos2x

why is tanx=2 wrong???
7. (Original post by justinawe)
Neither have I, but if you need examples you could always just google it
That's very true, I'll probably do that later on tonight

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8. (Original post by yaboy)
If tanxcos2x=2cos2x

why is tanx=2 wrong???
I'm presuming you divided through by cos2x?

Because you're dividing through by 0 if you divide everything by cos2x.
9. (Original post by yaboy)
If tanxcos2x=2cos2x

why is tanx=2 wrong???
tanxcos2x - 2cos2x = 0
cos2x(tanx-2)=0

you lost some solutions
10. (Original post by usycool1)
I'm presuming you divided through by cos2x?

Because you're dividing through by 0 if you divide everything by cos2x.
genuinely shocked that I never knew that before, first time ive ever had this issue trig
11. (Original post by Better)
Why are you not allowed to take cos of both sides.

f(x)= Cos(√x-2)-x+1

This is exactly what I put in my calculator thank you so much for checking this for me and your calculator looks incredible. How much did you get that for and will I be allowed to use it in the exam?
Yes of course its allowed for the exams.
Just check it on ebay. It's available there for \$119

Its TI 84
NOT TI 89 (TI 89 is NOT allowed)
12. Hey guys! Could someone please help me with this volume of revolution question?! I really can't do it!

A curve has the equation: y=cosec x
The finite region R is bounded by the curve, the x-axis and the lines x=pi/6 and x=pi/3

Find the exact volume of revolution, giving your answer in the form api√3 where a is a constant

I got as far as pi[3+2√3/3] but can't get it in the form they want!

Thankkss
13. (Original post by niki135)
Hey guys! Could someone please help me with this volume of revolution question?! I really can't do it!

A curve has the equation: y=cosec x
The finite region R is bounded by the curve, the x-axis and the lines x=pi/6 and x=pi/3

Find the exact volume of revolution, giving your answer in the form api√3 where a is a constant

I got as far as pi[3+2√3/3] but can't get it in the form they want!

Thankkss
You shouldn't have got the extra 3 in bold.
14. (Original post by StUdEnTIGCSE)
Yes of course its allowed for the exams.
Just check it on ebay. It's available there for \$119

Its TI 84
NOT TI 89 (TI 89 is NOT allowed)
That's alot of money....would you recommend it?

Like is it fast at drawing graphs and does it tell you intercepts?
15. (Original post by usycool1)
You shouldn't have got the extra 3 in bold.
OMG I did the wrong calculation! Just realised. I forgot to do y^2. I got the answer now Thank you! x
16. Not sure if I should buy a Graphics Calculator or not. I really need an A* in Maths.
17. (Original post by Better)
Not sure if I should buy a Graphics Calculator or not. I really need an A* in Maths.
There's no need in getting one as you don't really need one to be able to obtain an A*, just a waste of money in my eyes.

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18. (Original post by MathsNerd1)
There's no need in getting one as you don't really need one to be able to obtain an A*, just a waste of money in my eyes.

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Okay thanks. Guess its just me being dumb then. It would be dumb of me to think that every person who gets an A* has a graphics calculator.

Thanks for the quick response as always MathsNerd!
19. (Original post by MathsNerd1)
On a side note, yesterday my teacher printed out the information that told me where I lost my marks and I already knew about 9 that I lost but then I apparently lost 2 more as I can't differentiate anymore, I'm seriously going to take my time in this attempt to make sure nothing like that would happen again.

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Was this for C3 in January? It could've been worse... Just got my paper back, and found out I lost 2 marks on question 1 for subbing the value of x into the equation of the line, where the gradient should've been...What an idiot No stupid mistakes will be made this time round!

Anyway, can someone prod me in the right direction for this C4 vectors question? Part b. I know that r.p = 0... And that's about it I've found a = 18 and b = 9.

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20. (Original post by suncake)
Anyway, can someone prod me in the right direction for this C4 vectors question? Part b. I know that r.p = 0... And that's about it I've found a = 18 and b = 9.

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The point "P" lies on L1. So you can write the position vector of P in terms of lambda.

Then, the dot product of the direction vector of L1 and the position vector of P should equal 0....

Make sense?

See if you can go on.

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