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Let a, b, c be three distinct real numbers, none equal to one. If the vectors a i ˆ + j ˆ + k ˆ , + i ˆ + b j ˆ + k ˆ and i ˆ + j ˆ + c k ˆ are coplanar, then c 1 1 b 1 1 a 1 1      is equal to

1. 2
2. -1
3. -2
4. 1

Correct answer: 1

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