Toán 1) cho tan alpha=-2,π/2<2<π Tính cos alpha,sin alpha , cot alpha 30/09/2021 By Ximena 1) cho tan alpha=-2,π/2<2<π Tính cos alpha,sin alpha , cot alpha
Đáp án: \(\cot \alpha = – \dfrac{1}{2}\) Giải thích các bước giải: \(\begin{array}{l}1)Do:\dfrac{\pi }{2} < \alpha < \pi \\ \to \left\{ \begin{array}{l}\sin \alpha > 0\\\cos \alpha < 0\end{array} \right.\\\tan \alpha = – 2\\ \to \dfrac{{\sin \alpha }}{{\cos \alpha }} = – 2\\ \to \sin \alpha = – 2\cos \alpha \\{\sin ^2}\alpha + {\cos ^2}\alpha = 1\\ \to 4{\cos ^2}\alpha + {\cos ^2}\alpha = 1\\ \to {\cos ^2}\alpha = \dfrac{1}{5}\\ \to \cos \alpha = – \dfrac{1}{{\sqrt 5 }}\\ \to \sin \alpha = \dfrac{2}{{\sqrt 5 }}\\\cot \alpha = \dfrac{{\cos \alpha }}{{\sin \alpha }} = – \dfrac{1}{2}\end{array}\) Trả lời
Đáp án:
\(\cot \alpha = – \dfrac{1}{2}\)
Giải thích các bước giải:
\(\begin{array}{l}
1)Do:\dfrac{\pi }{2} < \alpha < \pi \\
\to \left\{ \begin{array}{l}
\sin \alpha > 0\\
\cos \alpha < 0
\end{array} \right.\\
\tan \alpha = – 2\\
\to \dfrac{{\sin \alpha }}{{\cos \alpha }} = – 2\\
\to \sin \alpha = – 2\cos \alpha \\
{\sin ^2}\alpha + {\cos ^2}\alpha = 1\\
\to 4{\cos ^2}\alpha + {\cos ^2}\alpha = 1\\
\to {\cos ^2}\alpha = \dfrac{1}{5}\\
\to \cos \alpha = – \dfrac{1}{{\sqrt 5 }}\\
\to \sin \alpha = \dfrac{2}{{\sqrt 5 }}\\
\cot \alpha = \dfrac{{\cos \alpha }}{{\sin \alpha }} = – \dfrac{1}{2}
\end{array}\)