Basic and Pythagorean Identities
Notice how a "co-(something)" trig ratio is always the reciprocal of some "non-co" ratio. You can use this fact to help you keep straight that cosecant goes with sine and secant goes with cosine.
Angle-Sum and -Difference Identities
![tan(2x) = [2 tan(x)] / [1 - tan^2(x)]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_vhVwXG9815HjXkIahH5EBUW3qMuq0wV4Dvcfn99yslPCakFI7dmGFywgo1eeUJIhrR3_9TctG-pDV0OArddw-RBpDT_Xlf1r4GjXOLn-FK3gjviH4FLg=s0-d)
Half-Angle Identities Copyright © Elizabeth Stapel 2010-2011 All Rights Reserved
![sin(x/2) = +/- sqrt[(1 - cos(x))/2], cos(x/2) = +/- sqrt[(1 + cos(x))/2], tan(x/2) = +/- sqrt[(1 - cos(x))/(1 + cos(x))]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_v6Q34QMaMvmwx4LWlMnEUb-HRcx0lwkgzij5ra52pK9KL_gip4RoqgPPt2IZKLed4r2Ub03-IZG2_qFteLcA3U22NxogABLweMrTMrVdnocFy3U1mZFw=s0-d)
The above identities can be re-stated as:
![tan^2(x) = [1 - cos(2x)] / [1 + cos(2x)]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_uL4tuxK6MtKBqRZxHySPdoOPrDwq8MEQyLQAEiBlnI2I9xHAeADjsGdQjQKWtGgSYAMkqT8VD_V8s62-5fo06cElj16FkUrKozcpVDCV_IBfXX5Chvmw=s0-d)
Sum Identities
![sin(x)+sin(y)=2sin[(x+y)/2]cos[(x-y)/2], sin(x)-sin(y)=2cos[(x+y)/2]sin[(x-y)/2], cos(x)+cos(y)=2cos[(x+y)/2]cos[(x-y)/2], cos(x)-cos(y)=-2sin[(x+y)/2]sin[(x-y)/2]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_uhe2yHbHQJM4y1OsTwHoHYcBWt4tUXg9aRUy105KZUo2Fy8Mx7ieVrL8Er2DGBNyr9oUJeIuMEqEt-mDUyuNJMtnnf3IGF82MU8SGLKz5P_9wEnks7=s0-d)
Product Identities
![sin(x)cos(y)=(1/2)[sin(x+y)+sin(x-y)], cos(x)sin(y)=(1/2)[sin(x+y)-sin(x-y)], cos(x)cos(y)=(1/2)[cos(x-y)+cos(x+y)], sin(x)sin(y)=(1/2)[cos(x-y)-cos(x+y)]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_vfbiYjRK7hLTnsXsCKn5ENtbQ1Ui7GMRnkSIALVsQnAi6IAglHwH2w1eqdfNyZjwRCLlJl0CATd2-wyKdAxPyYf4mh0mLY_gXlfuRMkEdM7XYgBdfbVg=s0-d)
sin2(t) + cos2(t) = 1 tan2(t) + 1 = sec2(t) 1 + cot2(t) = csc2(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.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)
![tan(a + b) = [tan(a) + tan(b)] / [1 - tan(a)tan(b)], tan(a - b) = [tan(a) - tan(b)] / [1 + tan(a)tan(b)]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_tEhpnapUzxq2zOC2ZExZuDyh-RP5R_PH9XoUPqrfaOVnxy7j5Hzab3hpS9Ls0VyfFFIMQq2lrNexo7UsR5vUqKc80v9Xx6hcRDt6qSabqJge2FcAc03A=s0-d)
Double-Angle Identitiessin(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)
sin(2x) = 2sin(x)cos(x)
cos(2x) = cos2(x) – sin2(x) = 1 – 2sin2(x) = 2cos2(x) – 1
sin2(x) = ½[1 – cos(2x)]
cos2(x) = ½[1 + cos(2x)]
