In \(\triangle ABC\), \(a=3,\ b=5,\ c=7\). Then angle \(C\) is
A. \(\frac{2\pi}{3}\)
B. \(\frac{5\pi}{6}\)
C. \(\frac{\pi}{3}\)
D. \(\frac{3\pi}{4}\)
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Answer: A. \(\frac{2\pi}{3}\)
\(\cos C=\frac{9+25-49}{2\cdot3\cdot5}=-\frac12\Rightarrow C=\frac{2\pi}{3}\).
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· 23 Oct 2026
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