You used the if operator incorrectly. The format is

if( condition, what to do if True, what to do if False) 

But you put another condition in the place of "what to do if false".

Here's an example: I want a function which is equal to $3x$ between $x=1$ and $x=4$, is equal to $x+8$ between $x=4$ and $x=5$, and is undefined otherwise. The code could be

if(x>5, ln(0), if(x>4, x+8, if(x>1, 3*x, ln(0) ) ) )

Here I'm using $\ln(0)$ with the hope that it will simply produce no output for the corresponding range, rather than halt the entire computation. Don't have the app (or iPad for that matter), so can't tell for sure.