Cos² 2x = (1+cos 4x )/2
cos² 6x = (1+cos 12x)/2
subtract to get 1/2 (cos 4x - cos 12x )
= 1/2 (2 sin 4x sin 8x ) as cos A - cos B = 2 [ sin (A+B)/2 sin (A-B)/2 ]
so the answer

we know that, the mixture amount is the sum of 11 + p, or
11 + p = m.
we also know that, the total value is also a sum of
25.3 + 4.5p = 3.29m.
Solve for the two velocities:
with the current = 48/6 = 8 mph
against the current = 48/8 = 6 mph
bv= boat velocity in still water
cv = current velocity
A) bv -cv = 6mph
B) bv + cv = 8mph
Adding both equations
2 * bv = 14 mph
boat velocity = 7 mph
current velocity = 1 mph
Calculator and tutorial can be found here:
http://www.1728.org/veloccal.htm