For non right angle triangles, finding a missing angle requires the use of either the sine rule of the cosine rule. Which one to use will depend on what information about the triangle you are given.

Use the sine rule if you have

  • an angle and its opposite side AND
  • another side

Otherwise use the cosine rule.

Once you have the sine or the cosine of the missing angle then you can the corresponding inverse ratio to find the angle.

Example using the Sine Rule to find a missing angle

Example using the Cosine Rule to find a missing angle

Find a missing angle - right angle triangle
nth Term of a Linear Sequence