05-10-09 Cos Ï€/5

Find the value of cos (Ï€/5)

4 Answers

(1+âˆš5)/4
here (1+âˆš5)/2= \phi
is the golden ratio
and cos(pi/5)= \phi/2

Arshad.. think of complex numbers.. there is a far better proof :)

Can we do it like this ??

Since e i ^Î¸ = cos ^Î¸ + i sin^Î¸

so cos Î¸ = (e ⁱ^Î¸ + e -^{i Î¸} ) / 2

yes.. now you can try.. you are on the right track..

I gave this one to teach this funda.. i mean more on how to use complex numbers to simplify otherwise the difficult looking subject trigonometry

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