Geometry · Grades 9 to 10
Angles and triangles
Angle sums and the sides of a right triangle.
The idea
The three angles inside a triangle always add to 180 degrees. When two angles are known, the third is the remainder.
In a right triangle, the Pythagorean theorem links the legs and the hypotenuse: a² + b² = c².
Worked examples
- How to Find a Missing Angle in a Triangle Find the third angle when two angles are 65 and 48 degrees. Three steps, the 180-degree reason, and the mistakes to avoid.
- How to Use the Pythagorean Theorem Find the hypotenuse of a right triangle with legs 6 and 8. Three steps, the theorem, and the square and root mistakes to avoid.
- How to Find the Area of a Circle Find the area of a circle with radius 5. Four steps with A = pi r^2, the exact answer in terms of pi, and the mistakes to avoid.
Practice this topic
4 problems with answers.