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vec ba n d vec c are unit vectors. Then ...

` vec ba n d vec c` are unit vectors. Then for any arbitrary vector ` vec a ,((( vec axx vec b)+( vec axx vec c))xx( vec bxx vec c))dot( vec b- vec c)` is always equal to `| vec a|` b. `1/2| vec a|` c. `1/3| vec a|` d. none of these

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`Let" " vecr=x_(1)veca+x_(2)vecb+x_(3)veccRightarrow vecr.(vecbxxvecc)=x_(1)veca.(vecbxxvecc)or x_(1)=([vecr vecb vecc])/([veca vecb vecc])`.
`Also, " " vecr.(veccxxveca)=x_(2)vecb.(veccxxveca)orx_(2)=([vecrveccveca])/([vecavecbvecc])`
`and vecr.(vecaxxvecb)=x_(3)vecc. (vecaxxvecb)or x_(3)=([vecr vecavecb])/([veca vecbvecc])`
`Rightarrow vecr=([vecrvecb vecc])/([vecavecbvecc])veca+([vecrveccveca])/([vecavecbvecc])vecb+([vecrvecavecb])/([vecavecb vecc])vecc`
`or [vecbveccvecr]veca+[veccvecavecr]vecb+[vecavecbvecr]vecr=[vecavecbvecc]vecr`
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