wtf i think i posted that comment in the wrong thread... LOL. either way, explanations for me can sometimes be a bit long winded. as a general rule of thumb:Take this guy's advice he's one of the top students for ext 1 and 2 at a school ranked in top 5!
if the question SPECIFICALLY SAYS "explain why", and it's one mark: it's an easy one mark but it's not one mark you want to lose i generally write as much as i can for these to make sure i get the marks, but never more than 5 lines worth. if you're writing so much for your explanations you're definitely doing something wrong.
IN PARTICULAR, i think it's important to note that your conclusions for longer questions which require lots of working don't need to be that long - just write "hence the coordinates of P are (acostheta, bsintheta)" if the question is "find the coordinates of P" or something like that.