A) \[60{}^\circ \]
B) \[100{}^\circ \]
C) \[90{}^\circ \]
D) \[120{}^\circ \]
E) \[140{}^\circ \]
Correct Answer: D
Solution :
Let \[a=3,b=5,c=7\] \[\therefore \] \[\cos C=\frac{{{3}^{2}}+{{5}^{2}}-{{7}^{2}}}{2\times 5\times 3}\] \[=\frac{9+25-49}{30}\] \[=-\frac{1}{2}\] \[=\cos 120{}^\circ \] \[\Rightarrow \] \[\angle C=120{}^\circ \]
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