(a + b + c)(a + b - c) = k ab: find angle C

1. A.
   $\frac{\pi}{2}$
2. B.
   $\frac{\pi}{4}$
3. C.
   $\frac{\pi}{3}$
4. D.
   $\frac{\pi}{6}$

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1. A.
   $\dfrac{\pi}{2}$
2. B.
   $\dfrac{2\pi}{3}$
3. C.
   $\dfrac{5\pi}{6}$
4. D.
   $\dfrac{3\pi}{4}$

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