Pythagoras’ Theorem a special case of the Cosine Rule

Trigonometry of the Triangle

Page 16 of the F&T booklet is divided into two halves. The top half gives the general formulae that apply to any triangle. The lower half gives the equivalent formulae for the special case of right-angled triangles.

If we note that cos 900 is zero then the cosine rule simplifies down to Pythagoras’ theorem.

If we note that the sin 900 is one then the sine rule simplifies down to our definition of sin = opposite over adjacent as used when dealing with right-angled triangles.

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