trigonometric identities m 120 precalculus v. j. motto
TRANSCRIPT
Trigonometric Identities
M 120 Precalculus
V. J. Motto
Preliminary Comments
• Remember an identity is an equation that is true for all defined values of a variable
• We are going to use the identities that we have already established to "prove" or establish other identities.
Let's summarize the basic identities we have.
Right Triangle Definitions
Unit Circle Definitions
Basic Trigonometric Identities
• Reciprocal Identities
Basic Trigonometric Identities
• Quotient Identities
Basic Trigonometric Identities
• Pythagorean Identities
Basic Trigonometric Identities
• Even-Odd Identities
22 sincoscscsin 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 sincoscscsin 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 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
sincotcsc
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
sincotcsc
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
• Find common denominators when there are fractions.
• Squared functions often suggest Pythagorean Identities.
• Work on the more complex side first.
• A denominator of 1 + trig function suggest multiplying top & bottom by conjugate which leads to the use of Pythagorean Identity.
• When all else fails write everything in terms of sines and cosines using reciprocal and quotient identities.
• Trigonometric Identities are like puzzles! They are fun and test you algebra skills and insights.
• Enjoy them! Attitude does make a difference in success.
Hints for Establishing Identities
Other Trigonometric Identities
• Identities expressing trigonometric function in terms of their complements.
Other Trigonometric Identities
• Sum formulas of sine and cosine
The derivation involves the use of geometry.
Other Trigonometric Identities
• Double angle formulas for sine and cosine
These are easily derived from the previous identities.