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Questions  >>  CBSE XII  >>  Math  >>  Model Papers
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Q)

Find the value of $\hat i\times (\hat j+\hat k)+\hat j\times (\hat k+\hat i)+\hat k\times (\hat i+\hat j)$

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A)
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  • $\hat i\times \hat i=\hat j\times\hat j=\hat k \times\hat k=0$
  • $\hat i\times\hat j=\hat k,\:\:\hat j\times\hat k=\hat i,\:\:\hat k\times\hat i=\hat j$
  • $\hat i\times\hat k=-\hat j,\:\:\hat j\times\hat i=-\hat k,\:\:\hat k\times\hat j=-\hat j$
$\hat i\times (\hat j+\hat k)=\hat i\times \hat j+\hat i\times \hat k$
$\qquad=\hat k-\hat j$
Similarly $\hat j\times (\hat k+\hat i)=\hat i-\hat k$
and
$\hat k\times(\hat i+\hat j)=\hat j-\hat i$
Adding all the three we get
$\hat i\times (\hat j+\hat k)+\hat j\times (\hat k+\hat i)+\hat k\times (\hat i+\hat j)$
$=\hat k-\hat j+\hat i-\hat k+\hat j-\hat i=\overrightarrow 0$
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