A) \[\pi /2\]
B) \[5\pi /6\]
C) \[2\pi /3\]
D) \[3\pi /4\]
Correct Answer: C
Solution :
Let sides of triangle \[a,b,c\]are respectively 3, 5 and 7. \[\therefore \cos C=\frac{{{a}^{2}}+{{b}^{2}}-{{c}^{2}}}{2ab}=\frac{9+25-49}{2\times 3\times 5}\]\[=\frac{-15}{30}=-\frac{1}{2}\] \[\angle C=\frac{2\pi }{3}\](largest angle).
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