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Q.

Let a,b and c be three non-coplanar vectors and d be a non-zero vector, which is perpendicular to a+b+c. Now, if d=(sinx)(a×b)+(cosy)(b×c)+2(c×a), then minimum value of x2+y2 is equal to

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a

π22

b

π24

c

π2

d

5π24

answer is D.

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Detailed Solution

a,b,c are non-coplanar [a,b,c]1 Also, a×b,b×c,c×a are non-coplanar given
d=sinx(a×b)+cosy(b×c)+2(c×a)
Taking dot product with a+b+c, we get
O=sinx[abc]+cosy[abc]+2[abc]
sinx+cosy+2=0   sinx+cosy=-2   x=(4n-1)π2,y=(2n-1)π,nz

for least value of x2+y2,x=-π2,y=π and least value is 5π24.

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