A necessary and sufficient condition for positive definiteness of a quadratic form Q(x) = x T Ax

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A necessary and sufficient condition for positive definiteness of a quadratic form Q(x) = xTAx with symmetric matrix A is that all the principal minors are positive that is,

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Show that the form in Prob. 22 is positive definite, whereas that in Prob. 23 is indefinite.

Data from Prob. 22

4x12 + 12x1x2 + 13x22 = 16

Data from Prob. 23

-11x12 + 84x1x2 + 24x22 = 156

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