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Question

Let a =i^+j^+k^, b =i^ and c ^=c1i^+c2j^+c3k^. Then,
(i) If c1 = 1 and c2 = 2, find c3 which makes a, b and c coplanar.

(ii) If c2 = −1 and c3 = 1, show that no value of c1 can make a, b and c coplanar.

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Solution

i If c1=1 and c2=2, then a=i^+j^+k^, b=i^ and c^=i^+2j^+c3k^.We know that vectors a, b, c are coplanar iff a b c=0.It is given that a, b, c are coplanar. a b c = 0 11110012c3=0 10-0-1c3-o+12-0=0-c3 + 2=0c3=2

ii If c2=-1 and c3=1, then a=i^+j^+k^, b=i^ and c^=c1i^-j^+k.^We know that vectors a, b, c are coplanar iff a b c=0.For a, b, c to be coplanar:a b c = 0111100c1-11=0 10-0-11-0+1-1-0=0-1-1=0-2=0But this is never possible, whatever be the value of c1. Thus, no vaue of c1 can make a, b and c coplanar.

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