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\(\cos a=\dfrac{-12}{13}\)
\(\sin b=\dfrac{4}{5}\)
\(\sin\left(a+b\right)=\sin a\cos b+\sin b\cos a\)
\(=\dfrac{5}{13}\cdot\dfrac{3}{5}+\dfrac{4}{5}\cdot\dfrac{-12}{13}=\dfrac{-45}{65}=\dfrac{-9}{13}\)
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\(\pi< a< \frac{3\pi}{2}\Rightarrow sina< 0\)
\(\Rightarrow sina=-\sqrt{1-cos^2a}=-\frac{12}{13}\)
\(sin2a=2sina.cosa=\frac{120}{169}\)
\(cos2a=2cos^2a-1=-\frac{119}{169}\)
\(tan2a=\frac{sin2a}{cos2a}=-\frac{120}{119}\)
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\(\frac{3\pi}{4}< a< \pi\Rightarrow\left\{{}\begin{matrix}sina>0\\cosa< 0\end{matrix}\right.\)
\(\left\{{}\begin{matrix}sin^2a+cos^2a=1\\2sina.cosa=-\frac{4}{5}\end{matrix}\right.\)
\(\Rightarrow\left\{{}\begin{matrix}sin^2a+cos^2a=1\\cosa=-\frac{2}{5sina}\end{matrix}\right.\)
\(\Rightarrow sin^2a+\frac{4}{25sin^2a}=1\)
\(\Leftrightarrow25sin^4a-25sin^2a+4=0\) \(\Rightarrow\left[{}\begin{matrix}sin^2a=\frac{4}{5}\\sin^2a=\frac{1}{5}\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}sina=\frac{2}{\sqrt{5}}\\cosa=-\frac{1}{\sqrt{5}}\end{matrix}\right.\\\left\{{}\begin{matrix}sina=\frac{1}{\sqrt{5}}\\cosa=-\frac{2}{\sqrt{5}}\end{matrix}\right.\end{matrix}\right.\)
Mà \(\frac{3\pi}{4}< a< \pi\Rightarrow\pi< a+\frac{\pi}{4}< \frac{5\pi}{4}\Rightarrow sina+cosa< 0\)
\(\Rightarrow\left\{{}\begin{matrix}sina=\frac{1}{\sqrt{5}}\\cosa=-\frac{2}{\sqrt{5}}\end{matrix}\right.\)
tại sao phải cộng thêm pi/4, mà tại sao cộng thêm pi/4 thì lại suy ra đc sina+cosa<0 vậy ạ
\(\dfrac{3\pi}{2}< a< 2\pi\Rightarrow sina< 0\)
\(\Rightarrow sina=-\sqrt{1-cos^2a}=-\sqrt{1-\left(\dfrac{3}{5}\right)^2}=-\dfrac{4}{5}\)
\(\Rightarrow sin2a=2sina.cosa=2.\left(-\dfrac{4}{5}\right).\left(\dfrac{3}{5}\right)=-\dfrac{24}{25}\)
Câu sau có nhầm đề ko nhỉ?
\(sin\left(\pi-\dfrac{\pi}{3}\right)=sin\left(\dfrac{2\pi}{3}\right)=\dfrac{\sqrt{3}}{2}\)