Write the following expression as a function of a positive acute angle. Use "+" to indicate a positive value and "-" to indicate a negative value.

As property of trigonometric cosine function, we have:

cos(x) = cos(-x)

=> cos(-240) = cos(240)

Otherwise, we also have:

cos(x + 180) = -cos(x)

=> cos(240) = - cos(60)

=> cos(-240) = cos(+240) = -cos(+60)

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