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📐 Finding Missing Angles

Use angle rules — straight lines, points, triangles and quadrilaterals — to work out missing angles.

2
lessons
~20 min
to learn
🔢 Math
subject
Ages 6–12
level
Start the course →

What you’ll learn

  1. Angles on Lines and PointsFind missing angles on a straight line (180°), around a point (360°), and using vertically opposite angles.Angles on a straight line add to 180° and angles around a point add to 360°, so subtracting finds a missing one. When two lines cross, vertically opposite angles are equal, and two angles inside a right angle add to 90°.
  2. Angles in ShapesFind missing angles inside triangles (180°) and quadrilaterals (360°) using a clear method.The angles in a triangle add to 180° and in a quadrilateral to 360°. To find a missing angle, name the rule for its total, add the angles you know, and subtract from the total. The same four-step method works every time.

Questions this course answers

Two angles on a straight line: one is 55°. The other is...

Angles on a line add to 180°, so 180 − 55 = 125.

Two lines cross. One angle is 65°. The angle vertically opposite it is...

Vertically opposite angles are equal, so it is also 65°.

A triangle has angles 80° and 25°. The third angle is...

A triangle's angles add to 180°: 80 + 25 = 105, and 180 − 105 = 75.

The four angles inside any quadrilateral add up to...

A quadrilateral's four angles always total 360°.

To find a missing angle on a straight line, which total do you use?

Angles on a straight line use the total 180°.

Three angles round a point are 100°, 140° and a missing angle. The missing angle is...

Round a point totals 360°: 100 + 140 = 240, and 360 − 240 = 120.

Grounded in trusted sources

  • Khan Academy Kids
  • BBC Bitesize KS2 Maths
  • NRICH Maths (Cambridge)
  • Britannica Kids
  • National Council of Teachers of Mathematics

Every Wunder lesson is built from real, reputable sources — never invented.

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