The facts

  • Angles on a straight line add up to 180°.
  • Angles round a point add up to 360°.
  • Vertically opposite angles (where two straight lines cross) are equal.
  • Angles in a triangle add up to 180°. An isosceles triangle has two equal base angles.
50°60°x

Here x = 180 − 50 − 60 = 70°.

Angles with x

If the angles on a line are 3x and (2x + 30)°, then 3x + 2x + 30 = 180, so 5x = 150 and x = 30. Check: 90 + 90 = 180.