Equation of a Circle

Applied Mathematics — Second Secondary

Common mistakes

Comparison aspect	Main idea	Example	Keywords
Mistake	writing the centre with the same signs as the coefficients in the general equation	(−2 3) instead of (2 −3) in Example (2). Correct: the centre is (−g −f) so the signs are changed	writingcentresamesigns
Mistake	writing the radius as r = g² + f² − c without the square root. Correct: r = √(g² + f² − c)	—	writingradiuswithoutsquare
Mistake	writing r in place of r² on the right-hand side of the standard form. Correct: the right-hand side is the square of the radius	—	writingplacerighthand
Mistake	declaring an equation to be a circle without checking that g² + f² − c is positive	—	declaringequationcirclewithout

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