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# p3 watch

1. sketch the curve:

x = 3cost y = 3sint

I just converted it into cartesian and then sketched that, is that right? Can someone post a pic of what it would look like(using one of those curve sketching programs) cheers.
2. (Original post by serious stuff)
sketch the curve:

x = 3cost y = 3sint

I just converted it into cartesian and then sketched that, is that right? Can someone post a pic of what it would look like(using one of those curve sketching programs) cheers.
Those eqns are the standard parametric eqns of a circle, centre the origin, of radius 3. Think of t as theta, an angle.
Look at the diagram attached. You have a circle with a point P on the circle. The x-coordinate and y-coordinate are given by,

x = OQ = OP.cosø
y = PQ = OP.sinø

OP is the radius of the circle, so you can put r=OP and write the parametric eqns as,

x = r.cosø
y = r.sinø

Bonus Material
=========
If it had been an ellipse, cutting the x-axis at a and the y-axis at b, then the parametric eqns for an ellipse would be,

x = a.cosø
y = b.sinø

giving,

x²/a² + y²/b² = 1
===========
Attached Images

3. Hey Fermat, what graph drawing program do you ue or recommend? I find graph drawing quite hard and have downloaded a 30day trial of Graph FAWM and have found that since I've been using it my ability has improved and so now I am thinking of investing in a program ready for University. Thanks
4. (Original post by Hoofbeat)
Hey Fermat, what graph drawing program do you ue or recommend? I find graph drawing quite hard and have downloaded a 30day trial of Graph FAWM and have found that since I've been using it my ability has improved and so now I am thinking of investing in a program ready for University. Thanks
I use Adobe Photo Deluxe, which is great for every thing except curves - circles, ellipse, parabolas - any curved shape.
To get around that I now use a program called DrawPlus 4.0, which is an old Serif program.
It's a bit finicky though. You can't save files as type .gif for example, you have to "export" them first. Other than that, it's not bad.
5. (Original post by Fermat)
I use Adobe Photo Deluxe, which is great for every thing except curves - circles, ellipse, parabolas - any curved shape.
To get around that I now use a program called DrawPlus 4.0, which is an old Serif program.
It's a bit finicky though. You can't save files as type .gif for example, you have to "export" them first. Other than that, it's not bad.
i tried to install that program but it doesnt work
6. Whats wrong with coverting it to cartesian and then sketching that?
7. (Original post by serious stuff)
Whats wrong with coverting it to cartesian and then sketching that?
Nothing!

x=3cost
y=3sint

If you square both sides and add them together you get,

x² + y² = 3²

which is the cartesian form (standard) of a circle, centre the origin, of radius 3.

I was simply pointing out, for those who might not recognise them, that those eqns were the standard parametrric eqns of a circle - knowing which, you can immediately see that what you have is a circle , centre the origin, of radius 3 - and so draw it straight away.
8. (Original post by tammypotato)
i tried to install that program but it doesnt work
Do you mean you didn't get it installed, or you couldn't get it to give you any circles etc.?
If the latter was the problem, then I had the same one!
Use the shape templates to draw a circle/ellipse. Now right-click on the red colour and click on "edit fill colour". Change the colour to white. Now alter the line size in the line info window on the rhs of the work screen.
9. (Original post by Fermat)
Nothing!

x=3cost
y=3sint

If you square both sides and add them together you get,

x² + y² = 3²

which is the cartesian form (standard) of a circle, centre the origin, of radius 3.

I was simply pointing out, for those who might not recognise them, that those eqns were the standard parametrric eqns of a circle - knowing which, you can immediately see that what you have is a circle , centre the origin, of radius 3 - and so draw it straight away.
OK cool!

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