Notice how a "co something " trig ratio is always the reciprocal of some "nonco" ratio. You can use this fact to help you keep straight that cosecant goes with sine and secant goes with cosine. The following particularly the first of the three below areed "Pythagorean" iden.ies..
Cos 2x = cos 2 x  sin 2 x = 1  x = x  1 HalfAngle Iden.ies The above iden.ies can be restated by squaring each side and doubling all of the angle measures..Tr.nometric functions. The secondary tr.nometric functions are the sine and cosine of an angle. These are sometimes abbreviated sin and cos , respectively, where is the angle, but the parentheses around . 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. You only need to know one, but be able to derive the other two from the Pythagorean formula..The Tr.nometric Iden.ies are equations that are true for Right Angled Triangles. The three main functions in tr.nometry are Sine, Cosine and Tangent..
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Verify Tr.nometric Iden.ies. How to verify tr.nometric iden.ies? Several examples with de.ed solutions are presented. Since we will make use of the basic .