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Angle facts
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A few simple angle facts let you find missing angles in all kinds of diagrams.
Types of angle
Acute: less than 90°. Right angle: exactly 90°. Obtuse: between 90° and 180°. Reflex: between 180° and 360°.
Angle facts
Angles on a straight line add up to 180°.
Angles around a point add up to 360°.
Vertically opposite angles (where two straight lines cross) are equal.
Angles around a point add up to 360°.
Vertically opposite angles (where two straight lines cross) are equal.
Triangles and quadrilaterals
Angles in a triangle add up to 180°.
An isosceles triangle has two equal base angles. Each angle in an equilateral triangle is 60°.
A quadrilateral splits into two triangles, so its angles add up to 2 × 180° = 360°.
An isosceles triangle has two equal base angles. Each angle in an equilateral triangle is 60°.
A quadrilateral splits into two triangles, so its angles add up to 2 × 180° = 360°.
An isosceles triangle has a top angle of 40°. Find each base angle.
- The base angles share 180° − 40° = 140°
- They are equal, so 140° ÷ 2
Answer: 70°
Straight line 180°, point 360°, vertically opposite angles equal, triangle 180°, quadrilateral 360°. Always give a reason.
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