Find without using tables, the values cos42° (cos18° - sin42° sin18°)

The given expression is in the form $\cos A \cos B - \sin A \sin B$. This matches the cosine addition formula:

$$\cos(A+B) = \cos A \cos B - \sin A \sin B$$

In this problem, $A = 42^\circ$ and $B = 18^\circ$.

$$\cos 42^\circ \cos 18^\circ - \sin 42^\circ \sin 18^\circ = \cos(42^\circ + 18^\circ)$$

We know that $\cos 60^\circ$ is a standard trigonometric value.

$$\cos 60^\circ = \frac{1}{2}$$

The final value is:

$$\cos 42^\circ \cos 18^\circ - \sin 42^\circ \sin 18^\circ = \boxed{\frac{1}{2}}$$