For your first question, the wording is a little strange and confusing but what I think they're talking about is that only the part that's left over after a multiple of is significant. For example, if you had something like , well that's just the same as because after you take out you have just 1 left.

For the second part, I'm confused as to how you got . It should be because

so, and

You take the value of theta that works for BOTH equations, in this case