complex numbers (1 Viewer)

[Hermes1]

w = cis 2pi/5

if w + 1/w = (-1 +/- root5)/2 find exact value of cos pi/5

this was is the last question of my half yearly

could some one tell me if my method is right

I got 2 cos 2pi/5 = ...
then i moved the two down
and used double angle

 

[xV1P3R]

w + 1/w = 2 cos 2pi/5, but 2pi/5 is acute.

So I think you disregard the negative number?

so 2cos 2pi/5 = [-1 + sqrt(5)]/2 only?

Double angle as you said, giving you

cos²(pi/5) = [3+sqrt(5)]/8

Square root then disregard the negative because pi/5 is acute as well.

 

[blackops23]

Square root then disregard the negative because pi/5 is acute as well.

ooooooh neeveer would have got that...