Explanation:

Trigonometry (from Greek trigōnon "triangle" + metron "measure") is a branch of mathematics that studies triangles and the relationships between their sides and the angles between these sides. Trigonometry defines the trigonometric functions, which describe those relationships and have applicability to cyclical phenomena, such as waves. It has applications in both pure mathematics and applied mathematics, where it is essential in many branches of science and technology.

http://en.wikipedia.org/wiki/Trigonometry

Complementary angle identities

Two angles whose sum is π/2 radians (90 degrees) are complementary. In the diagram, the angles at vertices A and B are complementary, so we can exchange a and b and change θ to π/2 − θ, obtaining:

(Our solved example in mathguru.com uses this concept).

http://en.wikipedia.org/wiki/Proofs_of_trigonometric_identities

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