trigonometric identities lecture
DESCRIPTION
Lesson Objectives Definition of Identity Enumerate Reciprocal Identities Enumerate Quotient Identities Could successfully analyze, verify and prove IdentitiesTRANSCRIPT
www.FroydWess.comPresents:
Trigonometric Identities
credit: Shawna Haider
Remember an identity is an equation that is true for all defined values of a
variable.
The left-hand expression always equals the right-hand expression, no matter what x equals.
xxx 2
RECIPROCAL IDENTITIES
sin
1cosec
cos
1sec
tan
1cot
QUOTIENT IDENTITIES
cos
sintan
sin
coscot
22 sec 1tan
22 cosec cot1
PYTHAGOREAN IDENTITIES
1 cossin 22
EVEN-ODD IDENTITIES
cotcotsecseccoseccosec
tantancoscossinsin
Note:
In every identities we can write every functions in terms of sine and cosine.
We can create different versions of many of these identities by using arithmetic.
22 sincoscosecsin Establish the following identity:
In establishing an identity you should NOT move things from one side of the equal sign to the other. Instead substitute using identities you know and simplifying on one side or the other side or both until both sides match.
22 sincoscosecsin Let's sub in here using reciprocal identity
22 sincossin
1sin
22 sincos1
We often use the Pythagorean Identities solved for either sin2 or cos2.
sin2 + cos2 = 1 solved for sin2 is sin2 = 1 - cos2 which is our left-hand side so we can substitute.
22 sinsin
We are done! We've shown the LHS equals the
RHS
cos1
sincotcosec
Establish the following identity:
Let's sub in here using reciprocal identity and quotient identity
Another trick if the denominator is two terms with one term a 1 and the other a sine or cosine, multiply top and bottom of the fraction by the conjugate and then you'll be able to use the Pythagorean Identity on the bottom
We worked on LHS and then RHS but never moved things
across the = sign
cos1
sincotcosec
cos1
sin
sin
cos
sin
1
cos1
sin
sin
cos1
combine fractions
cos1
cos1
cos1
sin
sin
cos1
2cos1
cos1sin
sin
cos1
FOIL denominator
2sin
cos1sin
sin
cos1
sin
cos1
sin
cos1
Get common denominators
If you have squared functions look for Pythagorean Identities
Work on the more complex side first
If you have a denominator of 1 + trig function try multiplying top & bottom by conjugate and use Pythagorean Identity
When all else fails write everything in terms of sines and cosines using reciprocal and quotient identities
Have fun with these---it's like a puzzle, you can use identities and algebra to get them to match!
Hints for Establishing Identities
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