Area is πr². Circumference is 2πr, or π times the diameter. Radius 4: area 16π, circumference 8π. Circle questions as a set are in circle problems.
Radius first
If you are given the diameter, halve it before you square. Diameter 10 is radius 5, so area is 25π, not 100π. Leave answers in terms of π unless the question asks for a decimal. 2πr and πd are the same circumference.
The coordinate form of a circle is in the equation of a circle. Arc length and sector area scale these two formulas, in arc length and sector area. Formulas as a set are in ACT Math formulas.
The bottom line
Square the radius for area. Do not square it for circumference.
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