Geometry Essentials: Angles, Area & Perimeter

Angles and triangles

Angles measure turn. Triangles are the most stable polygons — their angle sum is always 180°. Knowing one angle often unlocks the rest.

• Straight line: adjacent angles sum to 180°.
• Vertically opposite angles are equal.
• Triangle sum: ∠A + ∠B + ∠C = 180°.
• Pythagoras (right triangles): a² + b² = c² where c is the hypotenuse.

Unit awareness

Area uses square units; perimeter uses length units. Convert units before computing — mixing cm and m is a classic exam trap.