Tuesday, 3 March 2015

trigonometry - A Trig Identity

There are many trig identities that can be proven with known identities, but often the identities are tricky in finding the right methods to use. This is one of those identities that seems easy, but elusive. Consider the cubic equation
x314x2+56x56=0


for which one solution is 8sin2(2π/7) and is equal to 4.89008.... Now, another solution can be found to be
76cos(π/7)+6cos(2π/7)2cos(3π/7)

which is also found to have the numerical value 4.89008.... Since both forms have the same numerical value and satisfy the same equation they must be equal. This leads to the identity
8sin2(2π/7)=76cos(π/7)+6cos(2π/7)2cos(3π/7).



The purpose of this question is to ask what methods are best suited to prove this equation by use of trig identities, or a proof of this identity.

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