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🔎 Finding Missing Angles and Sides

Use angle rules and equal-side rules to work out missing angles and side lengths in shapes.

lessons

~20 min

to learn

🔢 Math

subject

Ages 6–12

level

Start the course →

What you’ll learn

Angles That Add UpUse angle rules to find missing angles: 180° on a line, 360° round a point, 180° in a triangle, and equal opposite angles.Angles on a straight line add to 180° and angles around a point add to 360°, so you can subtract to find a missing one. The three angles in any triangle add to 180°. When two lines cross, the angles opposite each other (vertically opposite) are equal.
Sides That MatchFind missing side lengths using the equal-side rules of squares, rectangles and equilateral triangles, and perimeter.A square has four equal sides and an equilateral triangle three, so one measurement gives them all. A rectangle's opposite sides are equal. If you know the perimeter of a square, divide by 4 to find one side.

Questions this course answers

Two angles sit on a straight line. One is 110°. What is the other?

Angles on a straight line add to 180°, and 180 − 110 = 70.

Angles around a point add up to how many degrees?

All the way around a point is a full turn, which is 360°.

A triangle has angles of 70° and 60°. The third angle is...

A triangle's angles add to 180°: 70 + 60 = 130, and 180 − 130 = 50.

When two straight lines cross, the angles opposite each other are...

Vertically opposite angles are always equal to each other.

A square has a perimeter of 24 cm. How long is one side?

A square has 4 equal sides, so 24 ÷ 4 = 6 cm.

A rectangle's top side is 9 cm. How long is its bottom side?

Opposite sides of a rectangle are equal, so the bottom is also 9 cm.

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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