Lesson 5.1
Using Trig Identities
• The textbook calls the
following the
Fundamental Trig
Identities
• They are down the left
side of your “Trig Cheat
Sheet”
• You will want to
memorize as many as
you can.
• Recognizing them
within an equation will
become important.
Cofunction Identities
These derive from a right triangle
With one angle 900 the other 2 angles must be complementary
Labeling the sides and setting up the 6 trig functions will show you
the cofunction identities.
u
c
a
900 - u
b
Cofunction Identities
Use the Unit Circle and ask the question:
“Would the value change if I plugged in the opposite value”
Biggest
challenge is to
know what to
use when!
Example
Verify the following:
sin  csc   sin 2   cos 2 
Example
Rewrite so it is not a fraction:
tan 2 x
csc x  1
Example
Verify the following:
1 + cos 𝜃
𝑠𝑖𝑛𝜃
+
= 2 csc 𝜃
𝑠𝑖𝑛𝜃
1 + 𝑐𝑜𝑠𝜃
Problem Set 5.1