3.1 Calculating Angles
If you get an exercise to calculate angles, you often base your calculations on the angle sum.

Example A
The triangle with the corners A, B and C are equal a ΔABC is equilateral.

The angle B is the triangle ABC is 60° a ΛB = 60°.

Example B
Here are two connected triangle’s. ΛBCA = 25°. Calculate ΛDEC.

ΛBCA = ΛECD = 25° (vertical angles).
ΛCDE is 90° (because it is a right angle).
ΛECD + ΛEDC + ΛDEC = 180º
ΛDEC is 180° - 90° - 25° = 65° (the angle sum in a triangle).