Khi tan so la $f1=60Hz => Z_{L1}=Z_{C1}$ (1)Khi tan so tang 2 lan len $f2=120Hz=> Z_{L2}=2Z_{L1}; Z_{C2}=\frac{Z_{C1}}{2}$
Ma khi tan so la $f2$ thi he so cong suat la $0.707\approx \frac{\sqrt{2}}{2}=> Z_{L2}-Z_{C2}=R=>2Z_{L1}-\frac{Z_{C1}}{2}=R$ (2)
Tu (1) va (2) co $Z_{L1}=\frac{2R}{3}=Z_{C1}$
Khi tan so la $f3=90Hz=> Z_{L3}=\frac{3}{2}Z_{L1}=R ; Z_{C3}=\frac{2}{3}Z_{C1}=\frac{4}{9}R$
Vay he so cong suat khi tan so la $f3=90Hz$ la
$cos\varphi =\frac{R}{\sqrt{R^{2}+(Z_{L3}-Z_{C3})^{2}}}=0,874$