Reduction formulas :

f (k * 90° +/- a ) = +/-g(a), k = 1,2,3,4

Where f and g are the names of trigonometric functions.

1. When k=1 or k=3, then g is the cofunction of function f (cofunction of sine is cosine and cofunction of tangent is cotangent, and vice versa). When k=2 or k=4 than g=f.

2. The sign, + or -, in front of function g is the same as is taken by function f in the appropriate quadrant.