Intermediate · 12 min
Angles and triangle area
Find missing angles and calculate area using a perpendicular height.
Use the angle sum
The interior angles of a triangle add to 180 degrees. Subtract the known angles to find the missing one.
Worked example
A triangle with angles 50° and 60° has a third angle of 70°.
Use the perpendicular height
A triangle's area is half its base times its perpendicular height. A sloping side is not usually the height.
Worked example
Base 8 cm and perpendicular height 5 cm give area 8 × 5 ÷ 2 = 20 cm².
Draw and measure
- Draw a triangle, choose a base, and mark the perpendicular height.
- Use your measurements to calculate its area.