The Student Room Group

Circle equations

When a question asks to write the equation of a circle should you write it like

(x2a)+(y2b)=r2 (x^2-a)+(y^2-b)=r^2

or

x2+y22ax2by+(a2+b2r2)=0 x^2+y^2-2ax-2by+ (a^2+b^2-r^2)=0 ?
Reply 1
You should write it like
(xa)2+(yb)2=r2(x-a)^{2}+(y-b)^{2}=r^{2}
It depends on the situation, but for most cases I would say the first form is more useful at a glance if you wanna know the center and radius straight away. Second form is better for algebraic manipulation and implicit diff

Your first layout is wrong BTW, squared should be outside the brackets
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(edited 9 years ago)
Reply 3
Okay, thanks guys :smile:

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