Mathematics for IGCSE & O level - Pythagoras' Theorem And Trigonometry (Section 3 - No. 13)
If a triangle has sides of 5 cm, 7 cm, and 9 cm, which rule is the most appropriate to determine the angles?
The Sine Rule
The Cosine Rule
Pythagorean Theorem
The Tangent Rule
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Since you know all sides, you can use the Cosine Rule to find the angles.