Solving Superposition Problem: Find a3 and Bint

[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?

 

[berkeman]

Try looking up some trig identities for sine functions. Is there one involving the addition of two sine waves and getting another sine wave with a phase shift?