Cho sina=-12/13;3π/2 30/09/2021 Bởi Elliana Cho sina=-12/13;3π/2 { "@context": "https://schema.org", "@type": "QAPage", "mainEntity": { "@type": "Question", "name": " Cho sina=-12/13;3π/2
$\frac{3\pi}{2}<a<2\pi$ $\Rightarrow cosa>0$ $\Rightarrow cosa=\sqrt{1-sin^2a}=\frac{5}{13}$ $cos(\frac{\pi}{3}-a)= cos\frac{\pi}{3}.cosa+sin\frac{\pi}{3}.sina$ $= \frac{1}{2}.\frac{5}{13}+\frac{\sqrt3}{2}.\frac{-12}{13}$ $= \frac{5-12\sqrt3}{26}$ Bình luận
$\frac{3\pi}{2}<a<2\pi$
$\Rightarrow cosa>0$
$\Rightarrow cosa=\sqrt{1-sin^2a}=\frac{5}{13}$
$cos(\frac{\pi}{3}-a)= cos\frac{\pi}{3}.cosa+sin\frac{\pi}{3}.sina$
$= \frac{1}{2}.\frac{5}{13}+\frac{\sqrt3}{2}.\frac{-12}{13}$
$= \frac{5-12\sqrt3}{26}$
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