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In mathematics, an "identity" is an equation i m sorry is constantly true. These have the right to be "trivially" true, choose "x = x" or usefully true, such as the Pythagorean Theorem"s "a2 + b2 = c2" for appropriate triangles. There are tons of trigonometric identities, however the complying with are the ones you"re most most likely to see and use.

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


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Notice exactly how a "co-(something)" trig proportion is always the mutual of some "non-co" ratio. You have the right to use this reality to aid you keep straight the cosecant goes v sine and secant goes through cosine.

The following (particularly the very first of the 3 below) are called "Pythagorean" identities.


Note the the three identities over all involve squaring and the number 1. You deserve to see the Pythagorean-Thereom relationship plainly if you take into consideration the unit circle, whereby the edge 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 concerned the useful status the the trig ratios:


Notice in particular that sine and also tangent room odd functions, gift symmetric around the origin, while cosine is an also function, being symmetric around the y-axis. The truth that you can take the argument"s "minus" sign exterior (for sine and tangent) or remove it entirely (forcosine) have the right to be advantageous when functioning 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)


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


By the way, in the above identities, the angles room denoted by 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 referred to as "beta", i m sorry is pronounce "BAY-tuh".

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

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


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


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

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


You will certainly be using every one of these identities, or nearly so, for proving various other trig identities and also for addressing trig equations. However, if you"re walking on to research calculus, pay details attention come the restated sine and cosine half-angle identities, since you"ll be using them a lot in integral calculus.