The Cosine Rule

This item is taken from IGCSE Mathematics (0580) Paper 43 of October/November 2010.

To deal with this item, let us summarize the details of the problem:
   (a) Syllabus area: Trigonometry
   (b) Specific topic: The Cosine Rule
   (c) Formula Needed:

Let us first identify the parts needed:

Since the unknown is angle PQR, let us write the cosine rule using PQR as the subject.

Let us now substitute the given values in the formula.

Simplify

We should take note that the problem requires us to show that angle PQR round to 110.7 degrees. This means that we should show the complete calculation for the angle and the full calculator display, which is 110.74238 degrees. This will verify that the rounded value of the angle is 110.7 degrees. 

Therefore, the measure of angle PQR is 110.7 degrees.

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