Answer: A) 120 degrees
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Step-by-step explanation:
Triangle ABC is equilateral, so all three sides are the same length. This means the three angles are also the same measure. Call each interior angle x.
The three angles add to 180, so x+x+x = 180 solves to x = 60. Each interior angle is 60 degrees.
Angle DAC is an exterior angle of measure 180-x = 180-60 = 120 degrees
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Or another method you could use is the fact that all exterior angles of any polygon add to 360.
Since we have n = 3 sides, this means each exterior angle of an equilateral triangle is 360/n = 360/3 = 120 degrees
We consider this a regular polygon because all sides are the same length and all angles are the same measure. So the formula for the exterior angle measure 360/n would apply to any regular polygon and not just triangles.