it is given that cos(π/5)=((1+(√5))/4) calculate the exact value of cos((2π)/5) and cos((3π)/5)


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Question Number 75193 by mathocean1 last updated on 08/Dec/19

$i t i s g i v e n t h a t c o s \frac{π}{5} = \frac{1 + \sqrt{5}}{4}$ $c a l c u l a t e t h e e x a c t v a l ue o f$ $c o s \frac{2 π}{5} a n d c o s \frac{3 π}{5}$
	Answered by $@ty@m123 last updated on 08/Dec/19

	Commented by $@ty@m123 last updated on 08/Dec/19

	$U s e f o r m u l a n o . 10 & 14 (b)$
	Commented by mathocean1 last updated on 08/Dec/19

	$thank you sir$
	Answered by Rio Michael last updated on 08/Dec/19

	$c o s 2 x = 2 {c o s}^{2} x - 1$ $\Rightarrow c o s \frac{2 π}{5} = c o s [2 (\frac{π}{5})] = 2 {c o s}^{2} \frac{π}{5} - 1$ $= 2 {[\frac{1 + \sqrt{5}}{4}]}^{2} - 1$
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