Angle between a and b when two combinations of a and b are perpendicular

1. A.
   $\frac{\pi}{6}$
2. B.
   $\cos^{-1}\frac{13}{14}$
3. C.
   $\frac{\pi}{3}$
4. D.
   $\cos^{-1}\left(-\frac{13}{14}\right)$

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1. A.
   $\frac{2\pi}{3}$
2. B.
   $\cos^{-1}\frac{2}{3}$
3. C.
   $\frac{\pi}{3}$
4. D.
   $\cos^{-1}\frac{1}{3}$

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1. A.
   $\frac{\pi}{3}$
2. B.
   $\frac{2\pi}{3}$
3. C.
   $\cos^{-1}\!\left(\frac{2}{3}\right)$
4. D.
   $\cos^{-1}\!\left(\frac{1}{3}\right)$

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1. A.
   $\frac{\pi}{4}$
2. B.
   $\frac{\pi}{3}$
3. C.
   $\cos^{-1}\!\left(\frac{1}{3}\right)$
4. D.
   $\cos^{-1}\!\left(\frac{3}{7}\right)$

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

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1. A.
   $\frac{\pi}{4}$
2. B.
   $\frac{\pi}{3}$
3. C.
   $\cos^{-1}\frac{1}{3}$
4. D.
   $\cos^{-1}\frac{2}{7}$

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

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