Trig identities: someone taught me this is in Year 10, it's the most useful thing I've ever learnt (EDIT: trig-wise...
) and I scribble it in pencil whenever I need it:
- draw three lines intersecting like an asterisk, with one horizontal, and the rest 60 degrees from each other
-from top row, label clockwise: sin cos cot cosec sec tan
-write '1' in the middle where the lines intersect
What this means:
-when you get an inverted triangle, sum of squares of top two is square of bottom
-across a line, each function (of x) is the same as the other function (of 90 - x)
-around the hexagon each function is the product of the two on each side
-opposites are reciprocals
Bloody useful. =) And if you have trouble, try working backwards, finding other ways of expression what you need to prove, etc.