prove sec2π7+sec22π7+sec23π7=24sec2π7+sec22π7+sec23π7=24 by using the roots of the polynomial x3−21x2+35x−7=0x3−21x2+35x−7=0
I tried to factor the polynomial but it didn't work and later found it cannot factorize with rationals. and I saw some similar problems in StackExchange. But the answers are very complex to me. I cannot use Euler's complex number formula since it's not in the syllabus. I do not want the exact answer but guidance to the answer.