Trigonometric identities

There are hundreds of them, if not thousands.

The first to remember is cos²x + sin²x = 1

If you have trouble remembering it, just think of Pythagoras on a simple triangle with sides 1, sin x and cos x.

Another useful one is tan x = (sin x)/(cos x)

Also remember what the co- means. The co-angle of a right angle triangle with angle θ is the other angle, which is 90-θ.

Thus, the cosine is the sine of the co-angle: cos x = sin 90-x

Similarly the cotangent is the tangent of the co-angle, and the cosecant is the secant of the co-angle.

There are also pairs of reciprocals: tan x = 1/cot x
sec x = 1/cos x
cosec x = 1/sin x

To remember, note that each pair has 1 co-.