Trigonometry of the right angled triangle. The definition of cosine, sine, and tangent of an angle.
Finding a side given one side and and angle. Calculating the angle given two sides.
This is a very important concept. Definition of sine, cosine, and tangent for angles greater than 90 degrees is through the unit circle. There is no modern trigonometry without the unit circle.
Under construction: this is the ambiguous case of the sine rule.
The ambiguous case solve using the cosine rule.