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Let k ˆ 4 j ˆ 3 i ˆ 2 a + + = G , k ˆ 2 j ˆ 2 i ˆ b − − = G and k ˆ 3 j ˆ 4 i ˆ c + + − = G . If d G is a vector perpendicular to both b G and c G , and d . a G G = 18, then 2 | d a | G G × is equal to-

1. 640
2. 680
3. 720
4. 760

Correct answer: 720

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