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# OCR C4 Parametric Equations watch

1. No idea what to do!! It's very irritating...

1) A curve has the parametric equations 'x= asin g' 'y= agcos g'

Where a is a positive constant and range is between (and including) pi to negative pi.

i) Write down the value of g corresponding to the origin, and state the points A and B (y and x intercept respectively of the curve).

ii) Show that dy/dx = -1gtan g and hence find the equation of the tangent to the curve at the origin.

I know how to do part ii) with the right information I just thought I should put it in for context, any help would be great cheers
2. I think you have to substitute in x=0 and y=0 to find the value of g but not totally sure what to do. sorry
3. No probs cheers for ur thoughts, ne1 else?
4. at origin (0,0), g = 0
5. How does that give you the two points then? I guess A is (O, pi) but I dunno, what about the second bit? Gah it's all such a pain!
6. at origin x = 0 ad y=0 so asing=0 and agcosg = 0 so g has to be 0 at the origin

at A x=0 so asing = 0 so g = 0 (y = 0) or g= pi (y=-pi) or g = -pi (y = pi)

at B y=0 so agcosg = 0 so g=0 (x=0) or g=pi/2 (x=pi) or g=-pi/2 (x = pi/2)

so A = (0,0) or (0,-pi) or (0,pi)
and B=(pi/2,0) or (0,0)
(from the diagram it should be obvious which one is B and which is A)

dy/dx = (dy/dg) / (dg/dy) (chain rule)
= (-agsing + acosg)/ acosg = -gtang + 1 (product rule on agcosg)

at origin g = 0 so dy/dx = 1
7. Here's the question in all its printed glory, hope this will help! Thanks for all your help so far by the way it's starting to make sense....
Attached Images

8. ive changed my post now hope it makes sence
9. Cool yeah cheers, pretty sure I got it, thanks for all you're help, sure it won't be the last time I post a maths question somehow!

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