P. 125, line 8

The line should be:

$c\cos \varphi =\sqrt{x_0^2+\frac{{\dot{x}}_0^2}{{\omega }_n^2}}\frac{x_0}{\sqrt{x_0^2+\frac{{\dot{x}}_0^2}{{\omega }_n^2}}},-c\sin \varphi =\sqrt{x_0^2+\frac{{\dot{x}}_0^2}{{\omega }_n^2}}\frac{x_0}{{\omega }_n\sqrt{x_0^2+\frac{{\dot{x}}_0^2}{{\omega }_n^2}}}.$

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