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Question

If a,bandc are non-coplanar vectors, then the following vectors are coplaner-

A
a+2b+3c,2a+3b4c,a3b+5c
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B
3a7b4c,3a2b+c,a+b+2c
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C
a2b+3c,2a+3b4c,ab+2c
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D
7a8b+9c,3a+20b+5c,5a+6b+7c
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Solution

The correct option is C 3a7b4c,3a2b+c,a+b+2c
For any three vectors to be co planar, their scalar triple product must be equal to zero.
Triple product can be found by writing down the numbers as a matrix and calculating its determinant.
Lets check option B,
3a7b4c3a2b+ca+b+2c
Now, lets find scalar triple product,
∣ ∣374321112∣ ∣=3[(2)(2)(1)(1)](7)[(3)(2)(1)(1)]4[3(1)1(2)]=3[41]+7[61]4[3+2]=3(5)+7(5)4(5)=15+3520=0
As the scalar triple product is zero, the three vectors
in option B are coplanar.

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