it-is-given-that-cos-pi-5-1-5-4-calculate-the-exact-value-of-cos-2pi-5-and-cos-3pi-5-

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