We need to find tan(1/2cos^-1(6/7)), which is. Let = . It means that is the angle in QI such that = . And we need to find = . According to the formulas of half-angle (they are basic formulas of Trigonometry; see the lesson Trigonometric functions of half argument in this site) = = = = , = = = = . Then = = = .
Prove the identity
{{{matrix(1,3,(1-2Cos^2x)/(Sin(x)*Cos(x)),"=",... (answered by Edwin McCravy)