"Why?" you ask. Imagine the unit circle if you will. Sine is positive in the first and second quadrant (between 0 and .) Now so lies in the second quadrant. By symmetry, it has the value of . Draw it out to see for yourself.
e) This one is trickier. Split it up into two parts. Let where
Now . Looking at your triangle the only angle that satisfies this equation is
So we now have . Look at your unit circle again and you'll realise that is the same thing as