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Let a r = i ˆ α + j ˆ – k and b r = i ˆ 2 + j ˆ – α k, α > 0. If the projection of a r × b r on the vector i ˆ – + j ˆ 2 –2k is 30, then α is equal to

1. 2 15
2. 8
3. 2 13
4. 7

Correct answer: 7

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