This doesn't seem to be true. Maybe there's a typo? Here's a counterexample.
Choose base 2, x=1 and y=4. The left hand side is
[tex]\log_2(x+y)=\log_2(1+4)=\log_2(5)[/tex]
The right hand side is
[tex]\log_2(1)+(1+\log_2(\frac{1}{4})) = 0+1-2=-1[/tex]
And they are not equal.
