When was Valentine supposedly executed? A: This was on February 14, 269 A.D.. B: This was on March 14, 269 A.D.. C: This was on July 7, 269 A.D.. D: This was on January 14, 269 A.D..

设\( {\bf{A}} \) 为三阶矩阵，\( { { \bf{A}}^*} \)是\( {\bf{A}} \)的伴随矩阵，且\( \left| {\bf{A}} \right| = 1 \)，则\( \left| {2 { { \bf{A}}^{ - 1}} + 3 { { \bf{A}}^*}} \right| = \)______

${\bf P}(X=4)=\,$ ${\bf P}(X=3)=\,$ ${\bf P}(X=2)=\,$ ${\bf P}(X=1)=\,$______

设${\bf{r}}$是从地心指向卫星质心的矢量，则表达式____总成立。 A: ${\bf{r}} \cdot {\bf{\dot r}} = r \cdot \dot r$ B: $\left| {{\bf{r}} \times {\bf{\dot r}}} \right| = r \cdot \dot r$ C: ${\bf{r}} \cdot {\bf{\ddot r}} = r \cdot \ddot r$ D: $\left| {{\bf{r}} \times {\bf{\ddot r}}} \right| = r \cdot \ddot r$

${\rm var}(X)=\,$ ${\bf E}[X]=\,$ ${\bf P}(X=3)=\,$ ${\bf P}(X=-2)=\,$ ${\bf P}(X=1)=\,$ ${\bf P}(X=0)=\,$______

设[img=11x14]180331b260e184b.jpg[/img]是[img=9x11]180331b269b44bf.jpg[/img]阶对称矩阵，[img=11x14]180331b2728fcba.jpg[/img]是[img=9x11]180331b269b44bf.jpg[/img]阶反对称矩阵，则（ ）是反对称矩阵。 未知类型：{'options': ['', '', '', ''], 'type': 102}

${\bf P}(X=-2)=\,$ ${\bf P}(X=1)=\,$ ${\bf P}(X=0)=\,$______

定义在（0，+∞）上的可导函数f（x）满足xf′（x）-f（x）＜0，则对任意a，b∈（0，+∞）且a＞b，有（） A: af（a）＞bf（b） B: bf（a）＞af（b） C: af（a）＜bf（b） D: bf（a）＜af（b）

《史晨碑》刻于公元()年。 A: 169 B: 269

${\bf P}(X=2)=\,$ ${\bf P}(X=1)=\,$______