Look at the following triangle and notice how it is conventionally labelled with the side of length **a **opposite the angle at** A**, the side of length **b** opposite the angle at **B**, and the side of length c opposite the angle at **C**.

The formulae for its area are:

Area = ½**ab**sin**C**

Area = ½**ac**sin**B**

Area = ½**bc**sin**A**

Choose the most appropriate formula, depending on the information that you have. You are looking for the given lengths of two sides and the angle between them.

**Example**

In the triangle above,

**b** = 4.3 cm

**c** = 5.1 cm

∠**A** = 38º

Use a trigonometrical formula, to calculate its area to 3 s.f.

**Answer**

Using the given information, we can only use the formula Area = ½ **bc**sin**A**.

Area = ½ x 4.3 x 5.1 x sin38º

**Area** = 0.5 x 4.3 x 5.1 x 0.616 = 6.7507... ≈ **6.75 cm ^{2} **(3 s.f.)