线性代数，第(3)小题，求矩阵的乘积，求高手，需要详细过程

Answer 1

很简单，
前两个矩阵相乘，得到
(x1a11+x2a12+x3a13 x1a12+x2a22+x3a23 x1a13+x2a23+x3a33)

然后用这个矩阵与最后的矩阵相乘，得到
x1(x1a11+x2a12+x3a13)
+x2(x1a12+x2a22+x3a23)
+x3(x1a13+x2a23+x3a33)
=
x1²a11+x2²a22+x3²a33 + 2a12x1x2+2a13x1x3+2a23x2x3