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Trigonometric Identities In mathematics, an "identity" is an equation which is always true. These can be "trivially" true, like "x = x" or usefully true, such as the Pythagorean Theorem's "a^{2} + b^{2} = c^{2}" for right triangles. There are loads of trigonometric identities, but the following are the ones you're most likely to see and use. Basic & Pythagorean, AngleSum & Difference, DoubleAngle, HalfAngle, Sum, Product Basic and Pythagorean Identities 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. sin^{2}(t) + cos^{2}(t) = 1 tan^{2}(t) + 1 = sec^{2}(t) 1 + cot^{2}(t) = csc^{2}(t) The above, because they involve squaring and the number 1, are the "Pythagorean" identities. You can see this clearly if you consider the unit circle, where sin(t) = y, cos(t) = x, and the hypotenuse is 1. sin(–t) = –sin(t) cos(–t) = cos(t) tan(–t) = –tan(t) Notice in particular that sine and tangent are odd functions, while cosine is an even function. AngleSum and Difference Identities sin(α
+ β) = sin(α)cos(β) + cos(α)sin(β)
DoubleAngle Identities sin(2x) = 2sin(x)cos(x) cos(2x) = cos^{2}(x) – sin^{2}(x) = 1 – 2sin^{2}(x) = 2cos^{2}(x) – 1 HalfAngle Identities Copyright © Elizabeth Stapel 20102011 All Rights Reserved The above identities can be restated as: sin^{2}(x) = ½[1 – cos(2x)] cos^{2}(x) = ½[1 + cos(2x)] Sum Identities Product Identities You will be using all of these identities, or nearly so, for proving other trig identities and for solving trig equations. However, if you're going on to study calculus, pay particular attention to the restated sine and cosine halfangle identities, because you'll be using them a lot in integral calculus.



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