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Angle facts and parallel lines
A handful of angle facts let you find almost any missing angle. Always give the reason.
Basic facts
Angles on a straight line add to 180°.
Angles around a point add to 360°.
Vertically opposite angles are equal.
Angles in a triangle add to 180°; in a quadrilateral, 360°.
Angles around a point add to 360°.
Vertically opposite angles are equal.
Angles in a triangle add to 180°; in a quadrilateral, 360°.
Parallel lines
Alternate angles are equal (a Z shape).
Corresponding angles are equal (an F shape).
Co-interior angles add to 180° (a C or U shape).
Corresponding angles are equal (an F shape).
Co-interior angles add to 180° (a C or U shape).
Triangles
An isosceles triangle has two equal sides and two equal base angles.
An equilateral triangle has three angles of 60°.
The exterior angle of a triangle equals the sum of the two opposite interior angles.
An equilateral triangle has three angles of 60°.
The exterior angle of a triangle equals the sum of the two opposite interior angles.
An isosceles triangle has a top angle of 40°. Find each base angle.
- 180 − 40 = 140
- The base angles are equal: 140 ÷ 2
Answer: 70°
Straight line 180°, point 360°, vertically opposite equal, triangle 180°. Parallel lines: alternate and corresponding equal, co-interior add to 180°.
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