When is the cosine rule needed?

For a non-right-angled triangle when you know two sides and the included angle, or all three sides and need an angle.

What happens to the cosine rule if the angle is 90°?

cos 90° is 0, so the formula collapses to Pythagoras. That is a check, not a reason to use it on every right-angled triangle.

Is the cosine rule on the formula sheet?

Often yes, in the seasons that provide a sheet. You still have to choose the right rearrangement and substitute the opposite side. Confirm this year’s sheet with school.

Cosine rule GCSE Maths questions — find a side or angle

Cosine rule GCSE Maths questionsCosine rule GCSE Maths questions: find a side from SAS, or an angle from three sides, in triangles with no right angle. Higher practice with marking.Start free GCSE Maths test · Pricing