Next Percentileby Hetvi Shah Tutoring
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Math · Circles

SAT circles: distinguish radius, area, and distance

The rule

The diameter is twice the radius. Circumference is 2πr, a distance around the circle. Area is πr², a measure of the region inside.

r = 5 mDiameter = 10 m
A circle has a radius of 5 meters from its center to its edge, and a diameter of 10 meters across the whole circle. The inside area is 25 pi square meters; the outside boundary is 10 pi meters long.

Worked example

A circular garden has diameter 10 meters. Find its area and the fence length needed to go around it.

  1. The radius is half the diameter: 10 ÷ 2 = 5 meters.
  2. The area is π × 5² = 25π square meters.
  3. The circumference is 2 × π × 5 = 10π meters. Keep the units distinct.

A common trap

Using 10 as the radius makes the area four times too large. Squaring belongs in the area formula, not the circumference formula.

Apply it, then revisit it

Try the skill in a separate question. After checking your answer, explain the step you used and return to any mistake later.

Choose Circles practice Open the lesson step