6.3.3 Cofunctions, Other Identities

• In some situations, you may know information pertaining to one trig identity/function, but not another– Ex. You know the cos of some angle ϴ, but not ϴ

itself– How would you find sin(ϴ)?

• Luckily, some trig functions are related to each other, thanks to the unit circle

• Cofunction Identities = equations to use for reference/simplifying particular expressions

Cofunctions

• If you know the angle ϴ in radians, then:

• cos(ϴ) = sin(π/2 - ϴ)

• csc(ϴ) = sec(π/2 - ϴ)

• cot(ϴ) = tan(π/2 - ϴ)

• Example. Express cos(5π/11) in terms of its cofunction.

• Example. Express cot(2100) in terms of its cofunction.

• Example. What is an equivalent trig identity to cos(870)?

• Example. What is an equivalent trig identity to sin(π/6)?

• Example. Given cot(ϴ) = 0.4, and ϴ lies in the fourth quadrant, determine sin(ϴ).– Hint. What ratio gives us cot(ϴ)?

• Assignment• Pg.498• 64-67 all• 69-77 odd