The line should be:
c cos ϕ = x 0 2 + x ˙ 0 2 ω n 2 x 0 x 0 2 + x ˙ 0 2 ω n 2 , − c sin ϕ = x 0 2 + x ˙ 0 2 ω n 2 x 0 ω n x 0 2 + x ˙ 0 2 ω n 2 . {\displaystyle c\cos \phi ={\sqrt {x_{0}^{2}+{\frac {{\dot {x}}_{0}^{2}}{\omega _{n}^{2}}}}}{\frac {x_{0}}{\sqrt {x_{0}^{2}+{\frac {{\dot {x}}_{0}^{2}}{\omega _{n}^{2}}}}}},\quad -c\sin \phi ={\sqrt {x_{0}^{2}+{\frac {{\dot {x}}_{0}^{2}}{\omega _{n}^{2}}}}}{\frac {x_{0}}{\omega _{n}{\sqrt {x_{0}^{2}+{\frac {{\dot {x}}_{0}^{2}}{\omega _{n}^{2}}}}}}}.}
Return to errata.
