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In mathematics, one "identity" is an equation which is always true. These deserve to be "trivially" true, favor "x = x" or usefully true, such as the Pythagorean Theorem"s "a2 + b2 = c2" for right triangles. There are loads of trigonometric identities, yet the following are the persons you"re most likely to see and also use.

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Basic & Pythagorean, Angle-Sum & -Difference, Double-Angle, Half-Angle, Sum, Product

Notice how a "co-(something)" trig proportion is always the reciprocal of part "non-co" ratio. You can use this fact to help you keep straight the cosecant goes with sine and secant goes v cosine.

The complying with (particularly the an initial of the 3 below) are dubbed "Pythagorean" identities.

Note the the three identities over all show off squaring and also the number 1. You deserve to see the Pythagorean-Thereom relationship clearly if you consider the unit circle, wherein the angle is t, the "opposite" side is sin(t) = y, the "adjacent" side is cos(t) = x, and also the hypotenuse is 1.

We have extr identities pertained to the practical status the the trig ratios:

Notice in details that sine and tangent room odd functions, being symmetric around the origin, when cosine is an also function, gift symmetric around the y-axis. The fact that you have the right to take the argument"s "minus" sign external (for sine and also tangent) or eliminate it entirely (forcosine) deserve to be useful when working with complicated expressions.

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### Angle-Sum and -Difference Identities

sin(α + β) = sin(α) cos(β) + cos(α) sin(β)

sin(α – β) = sin(α) cos(β) – cos(α) sin(β)

cos(α + β) = cos(α) cos(β) – sin(α) sin(β)

cos(α – β) = cos(α) cos(β) + sin(α) sin(β)

/ <1 - tan(a)tan(b)>, tan(a - b) = / <1 + tan(a)tan(b)>">

By the way, in the over identities, the angles are denoted through Greek letters. The a-type letter, "α", is referred to as "alpha", i m sorry is express "AL-fuh". The b-type letter, "β", is dubbed "beta", i beg your pardon is pronounced "BAY-tuh".

sin(2x) = 2 sin(x) cos(x)

cos(2x) = cos2(x) – sin2(x) = 1 – 2 sin2(x) = 2 cos2(x) – 1

/ <1 - tan^2(x)>">

, cos(x/2) = +/- sqrt<(1 + cos(x))/2>, tan(x/2) = +/- sqrt<(1 - cos(x))/(1 + cos(x))>" style="min-width:398px;">

The over identities deserve to be re-stated by squaring every side and also doubling every one of the angle measures. The results are together follows:

You will certainly be using all of these identities, or almost so, for proving other trig identities and for addressing trig equations. However, if you"re going on to examine calculus, pay specific attention come the restated sine and also cosine half-angle identities, because you"ll be making use of them a lot in integral calculus.