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A homogeneous polynomial of the second degree in `n` variables i.e., the expression `phi=sum_(i=1)^(n)sum_(j=1)^(n)a_(ij)x_(i)x_(j)` where `a_(ij)=a_(ji)` is called a quadratic form in `n` variables `x_(1),x_(2)`….`x_(n)` if `A=[a_(ij)]_(nxn)` is a symmetric matrix and `x=[{:(x_(1)),(x_(2)),(x_(n)):}]` then `X^(T)AX=[X_(1)X_(2)X_(3) . . . . .X_(n)][{:(a_(11),a_(12) ....a_(1n)),(a_(21),a_(22)....a_(2n)),(a_(n1),a_(n2)....a_(n n)):}][{:(x_(1)),(x_(2)),(x_(n)):}]` `=sum_(i=1)^(n)sum_(j=1)^(n)a_(ij)x_(i)x_(j)=phi` Matrix A is called matrix of quadratic form `phi`. Q. The quadratic form of matrix `A[{:(0,2,1),(2,3,-5),(1,-5,8):}]` isA. `3x_(2)^(2)+8x_(3)^(2)+2x_(1)x_(2)+x_(1)x_(3)-5x_(2)x_(3)`B. `3x_(2)^(2)+8x_(3)^(2)+4x_(1)x_(2)+2x_(1)x_(3)-10x_(3)x_(2)`C. `x_(1)^(2)+2x_(2)^(2)+x_(3)^(2)+3x_(1)x_(2)-5x_(2)x_(3)+8x_(1)x_(2)`D. `3x_(1)^(2)+8x_(2)^(2)+4x_(1)x_(2)+2x_(1)x_(3)+10x_(3)x_(2)` |
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Answer» Correct Answer - A `=[x_(1)x_(2)x_(3)][{:(2x_(2)+x_(3)),(2x_(1)+3x_(2)-5x_(3)),(x_(1)-5x_(2)+8x_(3)):}]` `=2x_(1)x_(2)+x_(1)x_(3)+2x_(1)x_(2)+3x_(2)^(2)-5x_(3)x_(2)+x_(3)x_(1)+5x_(3)x_(2)+8x_(3)^(2)` `=3x_(2)^(3)+8x_(3)^(2)+4x_(1)x_(2)+2x_(1)x_(3)-10x_(2)x_(3)` |
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