For any quadrilateral, the angles of the four corners add up to \(360^\circ\). The diagonals meet at right angles so we know we have an angle of \(90^\circ\). The three angles in a triangle add up ...

have three sides, therefore they have three angles. Interior angles in a triangle sum to a half turn (180˚). Quadrilaterals close quadrilateralA polygon with four straight sides. have four sides ...