If a, b and c are unit vectors satisfying | a - b|^2 + | b - c|^2 + | c - a|^2 = 9, then |2 a + 5 b + 5 c| is

JEE Advanced 2012 Mathematics question

If , and are unit vectors satisfying , then is

Answer: 3

Expanding with unit vectors: , so $ a· b + b· c +

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Question ID VK-026062