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
