Let →b and →c be two non-collinear vectors. If →a is a vector such that →a⋅(→b+→c)=4 and a×(→b×→c)=(x2−2x+6)→b+(siny)→c, then (x,y) lies on the line
A
x+y=0
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B
x−y=0
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C
x=1
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D
y=π
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Solution
The correct option is Cx=1 We know that, →a×(→b×→c)=(→a⋅→c)→b−(→a⋅→b)→c
Given, →a×(→b×→c)=(x2−2x+6)→b+(siny)→c ⇒(→a⋅→c)→b−(→a⋅→b)→c=(x2−2x+6)→b+(siny)→c
On comparing, we get →a⋅→c=x2−2x+6,→a⋅→b=−siny ∴→a⋅(→b+→c)=4⇒x2−2x+2=siny x2−2x+2=(x−1)2+1≥1 and siny≤1 ∴ Both sides are equal only for x=1