$(5$ points$)$ Consider the following harmonic functions:

$x_1(t)=5sin20t,$ and $,x_2(t)=8cos(20t+\frac{}{3})$

Express the function $x(t)=x_1(t)+x_2(t)$ as $($a$)$ a cosine function with a phase angle, and $($b$)$ a sine

function with a phase angle.