P. 404, lines 11 and 12

$y_{ss}=A\sqrt{[Re(G(i\omega )){]}^2+[Im(G(i\omega )){]}^2}\sin (\omega t+\varphi )$

and

$y_{ss}=A\left|G\left(i\omega \right)\right|\sin (\omega t+\varphi )$

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