※ 引述《polomoss (小澤)》之銘言：
: 1.Let A be m*n matrix with m>n and b be an m*1 vector
: then Ax=b may have multiple solution
: t
: 2.I+AA is invertible
: 3.A,B is positive definite,then so is A+B
3.
t t t
x(A+B)x = x A x + x B x > 0 ...
true
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