# Creating a Circle through an graph equation?

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On the graphing calculator I have, it doesn't come with drawing primitives and I seem to have forgotten the equation for a cirlce. e.g. y=(forgot what goes here).

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(x^2 + y^2)^0.5 = r
x^2 + y^2 = r^2
y^2 = r^2 - x^2
y = (r^2 - x^2)^0.5

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The parametric formula is

x = r sin(t)
y = r cos(t)

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ONE form of the Cartesian Equation for a circle is;

(x-a)^2 + (y-b)^2 = r^2

where (a,b) denotes the CENTRE of the circle and r denotes the RADIUS of the circle. (x, y) would denote the actual pixel plot on the screen.

The Parametric Equations of a circle, which for programming would perhaps be more useful, are;

x = rcosA + a
y = rsinA + b

where A denotes the ANGLE of the radial line.

Not really a whizz on this forum (but I sure know my math), cos I don't use it much... Otherwise, I'd show you a graphic that perfectly shows how these equations work.

One bit of optimization advice... If u need it that is... Is to try and use Trigonometric tables which hold the pre-computed values of cosA and sinA, as the angle A ranges from 0 - 360 degrees... Or 0 - 2*pi radians!

Hope this helps... If your still a bit confused... I'll try and send u a graphic :-)

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