Hi, For the following problem I am having hard time trying to find the phase angle (alpha).

How to find the phase angle,

isnt that alpha = arctan(B/A) if A,B>0 ?

Problem:

Solve the initial value problem

(where primes indicate derivatives with respect tot). Express your solution as the sum of two oscillations,(

xt) = (49/27)*cos(6t - ) + (5/27)cos(3t)

where=

(Recall that must be between 0 and 2.)

But the phase angle is not right.

Please help,

Many Thanks.