Finding amplitude and phase of superposed harmonic waves

[FoolishMortal]

The superposition of two harmonic waves:
u1 = B * sin( a1(r) )
u2 = B * sin( a2(r) )
results in a sinusoidal wave of the form:
uint = Bint * sin( a3(r) )
Find a3 and Bint

I'm not sure what to do. I can't think of any way to get it into that "form". http://scienceworld.wolfram.com/physics/Interference.html (5) on that link gives the form I would expect.
I think the "int" subscript is talking about intensity, and I'm not sure but I don't think either u1 or u2 depend on time.
Can anyone help?

 

[Meir Achuz]

Use your high school trig (ugh).
uint=u1+u2=B[sin(a)+sin(b)]=2B{sin[(a+b)/2]cos[(a-b)/2]}
if I remembered it right.