Maths · Trigonometry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The sine rule, the cosine rule and the area formula work in any triangle, not only right-angled ones. Label the angles A, B, C and call the side opposite each angle a, b, c.
Use the sine rule when you know a side and the angle opposite it, plus one more side or angle. Use the cosine rule when you know two sides and the angle between them, or all three sides.
The area formula needs two sides and the included angle, which is the angle between those two sides. The largest angle is always opposite the longest side. If the cosine of an angle comes out negative, the angle is obtuse.
In the cosine rule students work out (b2 + c2 − 2bc) first and then multiply by cos A. Only 2bc is multiplied by cos A: work out 2bc cos A, then subtract it from b2 + c2.
Answer: 6.57 cm
Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.
← Right-angled trianglesPythagoras’ theorem and trigonometry →Take a Maths test