The circle above with center O has a circumference of 36. What is the length of minor arc ?
9
12
18
36
Choice A is correct. A circle has 360 degrees of arc. In the circle shown, O is the center of the circle and is a central angle of the circle. From the figure, the two diameters that meet to form
are perpendicular, so the measure of
is
. Therefore, the length of minor arc
is
of the circumference of the circle. Since the circumference of the circle is 36, the length of minor arc
is
.
Choices B, C, and D are incorrect. The perpendicular diameters divide the circumference of the circle into four equal arcs; therefore, minor arc is
of the circumference. However, the lengths in choices B and C are, respectively,
and
the circumference of the circle, and the length in choice D is the length of the entire circumference. None of these lengths is
the circumference.