# Math Help - Rewriting a trig expression

1. ## Rewriting a trig expression

Write the expression as the sine, cosine, or tangent of an angle

Problem: cos (pi/2) cos(pi/7) + sin(pi/2) sin(pi/7)

I start with the cos(u - v) = cos( pi/2 - pi/7) =
(90 degrees - 25.7) < turned radians into degrees [pi/7 (180/pi)]=about 25.7
=
cos(64.3) = .433

2. Originally Posted by yeunju
Write the expression as the sine, cosine, or tangent of an angle

Problem: cos (pi/2) cos(pi/7) + sin(pi/2) sin(pi/7)

I start with the cos(u - v) = cos( pi/2 - pi/7) =
(90 degrees - 25.7) < turned radians into degrees [pi/7 (180/pi)]=about 25.7
=
cos(64.3) = .433
you were not asked to evaluate it. just write it as the cosine of an angle. you get $\cos \left( \frac {\pi}2 - \frac {\pi}7 \right) = \cos \frac {5 \pi}{14}$