Calculate the power of a discrete-time signal We got this question in our test :-
$$S(t) = 5\cos(10πt - π/2) + 6\sin(15πt)\cos(15πt)$$
My Solution :-
$$S(t) = 5\cos(10πt - π/2) + 3\sin(30πt)\qquad    \textrm{[Using $\sin(2A) = 2\sin(A)\cos(A)$]}$$
=&gt; Power$ = 5x5/2  +  3x3/2
         = 25/2 + 9/2
         = 17$
But in the solution, the correct answer was given to be 21.5 watt.
What mistake did I do?
Apart from possible dimensional problems as discussed in the comments, your result is correct and the given solution is wrong. In general with such exercises, don't forget to check if the cross-term really cancels out (in this case it does).