# Thread: Trigonometric Function

1. ## Trigonometric Function

Hi,
I don't understand this well.
It says:
use the value of the trigonometric function to evaluate the indicated functions.
cos t = 4/5
a) cos(pi - t)
b) cos(t + pi)

Can somebody help? Thank you
Isabel

2. Originally Posted by cuteisa89
Hi,
I don't understand this well.
It says:
use the value of the trigonometric function to evaluate the indicated functions.
cos t = 4/5
a) cos(pi - t)
b) cos(t + pi)

Can somebody help? Thank you
Isabel
we will use the double angle formulas for cosine, these are:

cos(A + B) = cosAcosB - sinAsinB
cos(A - B) = cosAcosB + sinAsinB

cos t = 4/5
a) cos(pi - t)
cos(pi - t) = cos(pi)cos(t) + sin(pi)sin(t) = (-1)(4/5) + 0 = -4/5

cos t = 4/5
b) cos(t + pi)
cos(t + pi) = cos(t)cos(pi) - sin(t)sin(pi) = (4/5)(-1) - 0 = -4/5

,

### use the value of the trigonometric function to evaluate the indicated

Click on a term to search for related topics.