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Question

For any three vectors a,b and c, prove that [a+b,b+c,c+a]=2[abc]. Hence prove that the vectors a+b,b+c,c+a are coplanar. If and only if a,b,c are coplanar.

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Solution


[(a+b).(b+c),(c+a)]=(a+b).[(b+c)×(c+a)]=(a+b).[b×c+b×a+c×c+c×a]=(a+b).(b×c+b×a+c×a)=a.(b×c)+0+0+0+0+b.(c×a)=[abc]+[bca]=[abc]+[abc]=2[abc]proved

now given vector will be coplanar only when a,b,carecoplanar,sothat[abc]=0


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