sin(x) = cos(90-x)
tan(x) = cot(90-x)
you want to find the cofunction of cos(2pi/7)
that would be sin(90 degrees - 2pi/7)
90 degrees is equal to pi/2, so you would get:
cos(2pi/7) = sin(pi/2 - 2pi/7).
pi/2 is equivalent to 7pi/14
2pi/7 is equivalent to 4pi/14
your equation becomes:
cos(2pi/7) = sin(7pi/14 - 4pi/14) which can be simplified to:
cos(2pi/7) = sin(3pi/14)
use your calculator to find cos(2pi/7) and sin(3pi/14).
