Câu hỏi 25

Sử dụng giản đồ vecto:

Từ giản đồ vecto ta thấy:

\(\left\{ \begin{array}{l}
{U_{L1}} = U.\sin {\varphi _1}\\
{U_{R1}} = U.\cos {\varphi _1}\\
{U_{L2}} = U.\sin {\varphi _2}\\
{U_{R2}} = U.\cos {\varphi _2}
\end{array} \right. \Rightarrow {U_{L1}} = \sqrt 2 {U_{L2}} \Leftrightarrow U.\sin {\varphi _1} = \sqrt 2 U.\sin {\varphi _2}\)

Vì:

\(\begin{array}{l}
{\varphi _1} + {\varphi _2} = \frac{\pi }{2} \Rightarrow \sin {\varphi _2} = \cos {\varphi _1} \Rightarrow U.\sin {\varphi _1} = \sqrt 2 U.\cos {\varphi _1}\\
\Rightarrow \tan {\varphi _1} = \sqrt 2 \Rightarrow \cos {\varphi _1} = \cos (\arctan \sqrt 2 ) = 0,577
\end{array}\)

Chọn B