Basic and Pythagorean Iden.ies. Note that the three iden.ies above all involve squaring and the number 1. You can see the Pythagorean-Thereom relationship clearly if you consider the unit circle, where the angle is t, the "opposite" side is sin t = y, the "adjacent" side is cos t = x, and the hypotenuse is 1..

Iden.ies for negative angles. Sine, tangent, cotangent, and cosecant are odd functions while cosine and secant are even functions. Ptolemy's iden.ies, the sum and difference formulas for sine and cosine. Double angle formulas for sine and cosine. Note that there are three forms for the double angle formula for cosine..Cos 2x = cos 2 x - sin 2 x = 1 - x = x - 1 Half-Angle Iden.ies The above iden.ies can be re-stated by squaring each side and doubling all .Tr.nometric functions. The functions sine, cosine and tangent of an angle are sometimes referred to as the primary or basic tr.nometric functions. Their usual abbreviations are sin , cos and tan , respectively, where denotes the angle..The Tr.nometric Iden.ies are equations that are true for Right Angled Triangles. Sine, Cosine and Tangent. The three main functions in tr.nometry are Sine, That is our first Tr.nometric Iden.y. Cosecant, Secant and Cotangent..