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Triangle 1
The angles of an equilateral triangle are equal.
and
The sum of the internal angles of a triangle is 180º.
180º/ 3 = 60º
∠ ABC = ∠ BCA = ∠ BAC= 60º
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Triangle 2
The sum of the internal angles of a triangle is 180º.
In isosceles triangles (with two equal sides) two of their angles are equal.
55º + x + ∠ 55º = 180º
∠ x = 180º - 110º
∠ x = 70º
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Triangle 3 (right triangle)
90º + ∠y + ∠ ECD = 180º
The sum of supplementary angles is180º
∠ ECD + x + ∠ BCA = 180º