A) \[{{\cos }^{-1}}(2/3)\]
B) \[{{\cos }^{-1}}(-2/3)\]
C) \[{{\tan }^{-1}}(2/3)\]
D) None of these
Correct Answer: A
Solution :
D.r?s are 2, 2, 1 and \[(7-3,\,\,2-1,\,\,12-4)\equiv (4,\,1,\,8)\] \ \[\cos \theta =\frac{2\times 4+2\times 1+1\times 8}{\sqrt{{{2}^{2}}+{{2}^{2}}+{{1}^{2}}}\sqrt{{{4}^{2}}+{{1}^{2}}+{{8}^{2}}}}=\frac{18}{9\times 3}=\frac{2}{3}\] Hence, \[\theta ={{\cos }^{-1}}(2/3)\].
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