Answer 5.2

The voltage output of the mixer used to compare the incoming symbol a(t) cos wct with the reference cos(wct + 25o) will be reduced by a factor cos(25o) or to 90.6% of its maximum value as a result of the carrier phase error.

This in turn equates to a reduction in symbol energy at the input to the receiver by a factor of cos2(25o = 0.82 or 0.85 dB. (Symbol energy is proportional to power x symbol length or voltage2 x symbol length).