Internal Angles of Square
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Theorem
The internal angles of a square are right angles.
Proof
By definition, a square is a regular quadrilateral.
From Internal Angles of Regular Polygon, the internal angles of a square measure $\dfrac {180 \degrees \paren {4 - 2} } 4 = 90 \degrees$.
The result follows from Measurement of Right Angle.
$\blacksquare$