5

5. [Maximum mark: 6]
   The following diagram shows triangle ABC, where AB = 4 cm, BC = 6 cm, AC = 5 cm and $\widehat{B}=2\theta$.

Find the exact value of $\cos\theta$, giving your answer in the form $\dfrac{p\sqrt{2}}{q}$, where $p, q \in \mathbb{Z}^+$. [6]