JAMB 1986 · UME · Q39

In $\triangle XYZ$ , $XY = 4$ , $XZ = 3$ and $YZ = 6$ . Determine the cosine of angle $Z$ .

Worked solution (try it first)

Angle $Z$ is between $XZ = 3$ and $YZ = 6$ .
The side facing it is $XY = 4$ .
Cosine rule: $\cos Z = \dfrac{3^2 + 6^2 - 4^2}{2 \times 3 \times 6}$ , which is $\dfrac{9 + 36 - 16}{36}$ .
So $\cos Z = \frac{29}{36}$ , option B.