You are Here: Home >< Maths

edexcel C3 question.HELP!!! watch

1. its on the january 2006 c3 paper question 4b

what does 8cos(arcsin(x/4)) convert to?

it shows that it can convert to 2(16-x^2)^0.5

i dont understand how this changes into this...someone explan please!!!
2. sin^2 x+cos^2 x=1
3. Are you sure it's January 2006? Cannot find this question.

EDIT:

Found it
4. (Original post by goldsilvy)
Are you sure it's January 2006? Cannot find this question.

EDIT:

Found it
What question. Can't find it.
5. (Original post by JTeighty)
What question. Can't find it.
It's on the MS. Question 4b.
Attached Images
6. C3Jan06A.pdf (636.5 KB, 70 views)
7. C3Jan06Q.pdf (154.2 KB, 208 views)
8. (Original post by goldsilvy)
It's on the MS. Question 4b.
What a stupid way to answer the question.

Just work out dx/dy and then take the inverse to make it dy/dx
9. (Original post by JTeighty)
What a stupid way to answer the question.

Just work out dx/dy and then take the inverse to make it dy/dx
That's what they did in the MS. Still there is this one final step,

For those interested,

10. x = 4 sin(2y + 6) -> x^2 = 16 sin^2(2y+6) -> (1-x^2)^0.5 = 4cos(2y + 6)

you have a 1/8cos(2y+6) so be careful with the constant multiplier on the bottom.
11. (Original post by goldsilvy)
That's what they did in the MS. Still there is this one final step,

For those interested,

Sorry didn't realise there was another stage.
12. Am really close. If you square the whole fraction you'll get 1/(64cos^2(arcsinx/4))
use sin^2+cos^2=1
so in the denominator u get 64-sin^2(arcsinx/4) which cancels to 64-x^2/16
Now root it.

Now I'm stuck but I think it's practically done
13. (Original post by JTeighty)
Am really close. If you square the whole fraction you'll get 1/(64cos^2(arcsinx/4))
use sin^2+cos^2=1
so in the denominator u get 64(1-sin^2(arcsinx/4) which cancels to 64(1-x^2/16)=64/16(16-x^2)=4(16-x^2)
Now root it.

Now I'm stuck but I think it's practically done
fixed
14. technically, since the answer's in brackets, it's an extra. You won't get marked down for not doing that, the question's not explicit enough.
15. (Original post by mathz)
fixed
Of course. What a silly mistake.

Thanks for correcting me :-)
16. how does the (2y+6) translate to arcsin(x/4)
17. someone pls help ... if so rep+
18. Because x=4sin(2y+6), you can undo this by dividing by 4, and undoing the sin, so that arcsin(x/4)=2y+6. After differentiation, you're asked to put dy/dx in terms of x, which means you can just substitute in the known value of 2y+6 into it's place in the equation.

Voila.
19. of course ... duh ... heres you rep

20. By making y the subject.

21. dude i'd give you a rep but just used ... 2moro thank btw
22. (Original post by morebritishthanindian)
how does the (2y+6) translate to arcsin(x/4)
if you use the orginal equation: x =4sin(2y+6)

x/4 = sin(2y+6)

arcsin(x/4) = 2y+6 then sub it back in to dy/dx

Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 20, 2010
Today on TSR

Results day under a month away

How are you feeling?

Poll
Useful resources

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

How to use LaTex

Writing equations the easy way

Study habits of A* students

Top tips from students who have already aced their exams

Chat with other maths applicants