20. Expressed as a single trigonometric function, $\cos ^{2}(6 m)-\sin ^{2}(6 m)$ is equal to: $2 \cos (3 m)$ $1-2 \cos (3 m)$ $\cos (12 m)$ $\sin (12 m)$

Step 1 ：The given expression is in the form of \(\cos^2(x) - \sin^2(x)\) which is equivalent to \(\cos(2x)\).

Step 2 ：So, we can replace \(x\) with \(6m\) to get the equivalent single trigonometric function.

Step 3 ：Final Answer: The equivalent single trigonometric function for \(\cos ^{2}(6 m)-\sin ^{2}(6 m)\) is \(\boxed{\cos (12 m)}\).