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Câu hỏi: Đặt điện áp $\text{u = U}\sqrt{\text{2}}\text{.cos }\omega\text{ t}$ vào hai đầu một đoạn mạch có R, L, C mắc nối tiếp thì hệ số công suất của đoạn mạch là
A. $\text{cos }\varphi\text{ = }\dfrac{\text{R}}{\sqrt{{{\text{R}}^{\text{2}}}\text{+ }{{\left( \omega\text{ L - }\dfrac{\text{1}}{\omega\text{ C}} \right)}^{\text{2}}}}}\text{.}$
B. $\text{cos }\varphi\text{ = }\dfrac{\text{R}}{\omega\text{ L - }{{\left( \omega\text{ C} \right)}^{-1}}}\text{.}$
C. $\text{cos }\varphi\text{ = }\dfrac{\left| \omega\text{ L -} \omega\text{ C} \right|}{\sqrt{{{\text{R}}^{\text{2}}}\text{+ }{{\left( \omega\text{ L - }\dfrac{\text{1}}{\omega\text{ C}} \right)}^{\text{2}}}}}\text{.}$
D. $\text{cos }\varphi\text{ = }\dfrac{\omega\text{ L + }\omega\text{ C}}{\text{R}}\text{.}$
A. $\text{cos }\varphi\text{ = }\dfrac{\text{R}}{\sqrt{{{\text{R}}^{\text{2}}}\text{+ }{{\left( \omega\text{ L - }\dfrac{\text{1}}{\omega\text{ C}} \right)}^{\text{2}}}}}\text{.}$
B. $\text{cos }\varphi\text{ = }\dfrac{\text{R}}{\omega\text{ L - }{{\left( \omega\text{ C} \right)}^{-1}}}\text{.}$
C. $\text{cos }\varphi\text{ = }\dfrac{\left| \omega\text{ L -} \omega\text{ C} \right|}{\sqrt{{{\text{R}}^{\text{2}}}\text{+ }{{\left( \omega\text{ L - }\dfrac{\text{1}}{\omega\text{ C}} \right)}^{\text{2}}}}}\text{.}$
D. $\text{cos }\varphi\text{ = }\dfrac{\omega\text{ L + }\omega\text{ C}}{\text{R}}\text{.}$
$\text{cos }\varphi\text{ = }\dfrac{R}{Z}=\dfrac{\text{R}}{\sqrt{{{\text{R}}^{\text{2}}}\text{+ }{{\left( \omega\text{ L - }\dfrac{\text{1}}{\omega\text{ C}} \right)}^{\text{2}}}}}\text{.}$
Đáp án A.