Write the sum as a product:

$cos(8\mathrm{.}6t)$

Note: coefficients inside of the trigonometric functions should be entered as $($simplified$)$ decimal valuesdo not enter fractions inside of the trig functions...else they will be marked incorrect by the system. For example, if your answer is $\frac{11}{2},$ enter $5\mathrm{.}5$ instead of $\frac{11}{2}.$