30sin( alpha + beta ) = 30*sin( alpha + beta ) =

= 30*( sin(alpha)*cos(beta) + cos(alpha)*sin(beta) )

the same thing with 50

sin( alpha + beta ) is only one number just like 30 or 50 so if you are multiplying it is the same how you will write sin( alpha + beta ).

For example:

a = a

b = c + d

a*b = a*( c + d )