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**angle**(degrees) 90° (for square and rectangle) In Euclidean plane geometry, a**quadrilateral**is a polygon with four edges (or sides) and four vertices or corners.Moreover, how do you find the missing angle of a quadrilateral?

Step 1: Add together the

**measures**of the known**angles**. Step 2: Subtract the sum from 360° to**determine**what remains for the fourth**angle**. The**measure**of the unknown**angle**is 145°.Do opposite angles in a quadrilateral equal 180?

**Opposite angles**in a cyclic

**quadrilateral**add up to

**180**° A cyclic

**quadrilateral**is a

**quadrilateral**whose vertices all touch the circumference of a circle. The

**opposite angles**add up to

**180**

^{o}. In the cyclic

**quadrilateral**below,

**angles**A + C =

**180**

^{o}, and

**angles**B + D =

**180**

^{o}.

Do all quadrilaterals have 360 degrees?

The

**Quadrilateral**Sum Conjecture tells us the sum of the angles in any convex**quadrilateral is 360 degrees**. Remember that a polygon**is**convex if each of its interior angles**is**less that 180**degree**.