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Circumference of a circle
C=2πr


Interior angles of a triangle add up to...
180°

An isoceles triangle has how many congruent sides?
2

If a triangle has two congruent sides, then the angles opposite the two sides are congruent.
ABC: since both angle A and angle C have measure 50° it follows that AB=BC. Also, since 50+50+x=180, the measure of angle B is 80°.


Area of a parallelogram
A=bh


The measure of an arc is the measure of its central angle, which is the angle formed by two radii that connect the center of the circle to the two endpoints of the arc. An entire circle is considered to be an arc with measure 360°.
The measure of arc ABC is 50° and the measure of arc ADC is 310°.

The ratio of the length of an arc to the circumference is equal to the ratio of the degree measure of the arc to 360°.
Length of arc to circumference is as degree of arc is to 360°.
The circumference of the circle is 10π.Therefore,


Just as in the case of the length of an arc, the ratio of the area of a sector of a circle to the area of the entire circle is equal to the ratio of the degree measure of its arc to 360°.

