Double Angle formula can transform the circular function equation to a biquadratic algebraic equation, and the analysis solution can be solved by extract equation directly.
用倍角公式可将该三角函数方程转化成一个四次代数方程，然后用求根公式直接求出解析解。
2
In chapter 3, a one-dimensional isotropic biquadratic Heisenberg spin chain equation is considered.