Grade 7 › G7U07 Circles & Area

Grade 7 › G7U07 Circles & Area › L1
GRADE 7 · ALBERTAOUTCOME AB-G7-SS1

L1 · Radius, diameter and circumference

Learning goal

“I can name the centre, radius, diameter and circumference of a circle, I know the diameter is exactly two radii (d = 2r), and I know the circumference is a little more than 3 times the diameter.”

Centre — the point in the exact middle of a circle; every point on the circle is the same distance from it.
Radius — a line segment from the centre to any point on the circle. Its length is also called the radius (r).
Diameter — a line segment that passes through the centre and touches the circle at both ends (d). A diameter is exactly two radii laid end to end.
Circumference — the distance all the way around the circle (C) - its perimeter.
Worked example

A circle has centre O. A segment from O to the edge measures 6 cm. Step 1: That segment runs from the centre to the circle - it is a radius: r = 6 cm. Step 2: Extend the segment straight through O until it touches the circle on the far side. That full segment is a diameter, and it is made of two radii in a straight line. Step 3: So d = 2 x r = 2 x 6 = 12 cm. Say it plainly: the radius is HALF the diameter; the diameter is DOUBLE the radius. Going r -> d means multiply by 2. Going d -> r means divide by 2.

Check yourself — explain it

A diameter must...

Paired problem — example still visible

A circle has radius 11 cm. Its diameter? -> 22 cm (2 x 11)

Faded completion — last step is yours

"A circle has diameter 20 cm. Its radius is 20 / ___ = ___ cm" -> 20 / 2 = 10 cm

Ready to practise?

10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.

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