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The equilateral triangle shown is rotated about line a. Each side of the triangle measures 20 mm.

An equilateral triangle is shown. Line of rotation a goes through one point and the midpoint of the opposite side.

What shape is created by the rotation and what is the approximate circumference of the base?

Circumference of a circle: C = 2πr

a cylinder with a circumference of about 63 mm
a cylinder with a circumference of about 126 mm
a cone with a base circumference of about 63 mm
a cone with a base circumference of about 126 mm

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Answered by GPT-5 mini AI
A cone with a base circumference of about 63 mm.

Reason: Rotating the equilateral triangle about the median through the vertex produces a cone. The base radius is half the side = 20/2 = 10 mm, so C = 2πr = 2π(10) = 20π ≈ 62.8 ≈ 63 mm.