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Explanation:
Focus on the bottom most triangle. The upper angle of this triangle is 360/5 = 72 degrees. This is because the regular pentagon is split into five congruent (aka identical) triangles.
Each of the triangles are isosceles. Let x be the measure of the base angles.
Use the rule that adding the three angles of a triangle gets us 180
angle1+angle2+angle3 = 180
x+x+72 = 180
2x+72 = 180
2x = 180-72
2x = 108
x = 108/2
x = 54 is the answer
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Another way to get the answer:
i = interior angle of a regular polygon with n sides
i = 180(n-2)/n
i = 180(5-2)/5
i = 180(3)/5
i = 540/5
i = 108
Each interior angle of a regular pentagon is 108 degrees.
Angle x marked in blue is half that, so it is 108/2 = 54 degrees
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A third method:
E = exterior angle of a regular polygon with n sides
E = 360/n
E = 360/5
E = 72 degrees is the measure of each exterior angle
i = 180 - E
i = 180-72
i = 108 is the measure of each interior angle.
To wrap things up, we follow the same exact steps at the end of the 2nd section to get 108/2 = 54 as our answer.