Circle Anatomy and Core Attributes Answer Key

Worksheet 1 of 5 in the Grade 7 circles series. Name the parts of a circle and move between radius and diameter. Next: The Meaning of Pi.

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Lesson Notes

A circle is the set of all points that are the same distance from one fixed point, the center. Every other part of a circle is named by how it relates to the center.

The diameter is two radii laid end to end through the center.

Key vocabulary

Radius ($r$): a segment from the center to any point on the circle. Every radius of a circle is the same length.
Diameter ($d$): a segment from one point on the circle to another that passes through the center. It is the longest segment (chord) you can draw inside a circle.
Chord: any segment with both endpoints on the circle. A diameter is a special chord.
Circumference ($C$): the distance around the circle (its perimeter). You will use it in the next worksheets.

The one relationship to memorize

$d = 2r$$r = d \div 2$

If you know one, you can always find the other. Doubling the radius gives the diameter; halving the diameter gives the radius.

Worked example

A circle has a diameter of 15 cm. Find the radius.

$r = d \div 2 = 15 \div 2 = 7.5$ cm

Watch for the words. "Across," "from edge to edge," and "the widest part" all describe a diameter. "From the center," "spray distance," and "rope length" all describe a radius.

25 problems. Work each one on the canvas under it (pen, eraser and clear are in the canvas toolbar; drag the handle below the canvas for more room). Enter your answer in the box (the keypad under it has fractions, exponents, square roots, pi, the degree sign and the symbols < > ≤ ≥ ≠). When a Unit row is shown, tap the unit for your answer. Then press Check. No $\pi$ is needed on this worksheet. The eye at the right of the row shows or hides the worked solution.