Hello, I dont know wether I've posted this in the right section but im really stuck on this question. Its from a signal processing exam paper:

Show that the signal is x(t) + y(t) is a sinusoid and determine its amplitude, frequency and phase.

The answer on the answer paper is:

x(t) + y(t) = cos (pi/2) t + sin (pi/2) t

= √2 cos ((pi/2) t - pi/4)

I have no idea how this is the answer. Am I missing something? can anyone show me the FULL working out?