ANSWER:
2sin(30)cos(30)
EXPLANATION:
Recall the below double angle trigonometric identities;
[tex]\begin{gathered} \cos2A=\cos^2A-\sin^2A \\ \\ \cos2A=2\cos^2A-1 \\ \\ \cos2A=1-2\sin^2A \end{gathered}[/tex]
So if we're given cos 60, which is the same as cos 2(30), where A is 30 degrees, so the following expressions can be used to determine the exact value of cos 60;
identities;
[tex]\begin{gathered} \cos2(30)=\cos^230-\sin^230 \\ \\ \cos2(30)=2\cos^230-1 \\ \\ \cos2(30)=1-2\sin^230 \end{gathered}[/tex]
So the only expression that cannot be used to determine the exact value of cos 60 2sin(30)cos(30)
