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Direction cosine l for ratios a, b, c is given by l =
Aa(a² + b² + c²)
Ba/(a + b + c)
C±a/√(a² + b² + c²)
D±a²/(a² + b² + c²)
Answer & Solution
Correct answer: C. ±a/√(a² + b² + c²)
Setting l = ka with l² + m² + n² = 1 gives k = ±1/√(a² + b² + c²).
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