This is a free lesson from our course in Trigonometry

 This lesson introduces the concept of Pythagorean trigonometric identities. The instructor will explain it to you in his/her own voice and handwriting in the video with some examples that will help you to understand the concepts and to improve your study skills.The Pythagorean trigonometric identity is a trigonometric identity expressing the Pythagorean Theorem in terms of trigonometric functions. Along with the sum-of-angles formulae, it is the basic relation among the sin and cos functions from which all others may be derived. For example: let's look at the relationship between one of the important identity sin2  + cos2  = 1 and the Pythagorean Theorem. (More text below video...)
Other useful lessons:
 Verifying Trigonometric Identities Getting started - Trigonometric identities
(Continued from above) The Pythagorean Theorem states that if the legs of a right triangle are a and b, and hypotenuse is c, then
(Hypotenuse)2 = (Adjacent)2 + (Opposite)2   i.e., c2 = a2 + b2.
In a right triangle, x and y are the legs, hypotenuse r, and be the measure of the central angle. Then the values of the six trigonometric functions are given by:
sin = opposite/hypotenuse =  y/r,
cos  = adjacent/ hypotenuse = x/r