Using Euler's Formula, show that $y=C_1{e}^{i(t+)}+C_2{e}^{-i(t+)}$ is equivalent to $y$

$=$Acos$(t+)$ for a judicious choice of $C_1,C_2$ and $A.$ In fact, derive a single

mathematical relationship between $C_1,C_2$ and $A.$