Latex Tests: Solving for Phi(a, theta) and Phi(b, theta)

[fluidistic]

I haven't found a thread for latex tests, but I'm sure there's at least one of them. So feel free to move this post.
$\begin{array} \Phi (a, \theta ) = \sum _{l=0}^ \infty [A_l a^l + B_l a^{-(l+1)}] P_l (\cos \theta ) = V_a \\ \Phi (b, \theta ) = \sum _{l=0}^ \infty [A_l b^l + B_l b^{-(l+1)}] P_l (\cos \theta ) =V_b \end{array}$
I don't understand why the first lines does not read "Phi(a, theta)". I wrote \Phi (a, \theta ).
Anyone has an idea? It works for the second line.

 

[Borek]

Perhaps first argument after {array} is always treated as a column definition? Not that \Phi would made any sense in this context...

$$\begin{array}{ll} \Phi (a, \theta ) = \sum _{l=0}^ \infty [A_l a^l + B_l a^{-(l+1)}] P_l (\cos \theta ) = V_a \\ \Phi (b, \theta ) = \sum _{l=0}^ \infty [A_l b^l + B_l b^{-(l+1)}] P_l (\cos \theta ) =V_b \end{array}$$

 

[fluidistic]

I see. Thanks a lot, I was not aware of this.