For what value of $x$ is the following true? \[ \log (x+9)=\log x+\log 9 \]

Step 1 ：The equation given is \(\log (x+9)=\log x+\log 9\).

Step 2 ：The property of logarithms that can be used to solve this equation is that the logarithm of a product is the sum of the logarithms of the individual factors.

Step 3 ：Therefore, we can combine the right side of the equation using this property to get \(\log (x+9)=\log (9x)\).

Step 4 ：Then, we can remove the logarithms from both sides of the equation by exponentiating to get \(x+9=9x\).

Step 5 ：This simplifies to \(x=\frac{9}{8}\).

Step 6 ：So, the solution to the equation is \(x=\frac{9}{8}\).

Step 7 ：Final Answer: The value of \(x\) for which the equation is true is \(\boxed{\frac{9}{8}}\).