[问答题]
The position vectors of the points A, B and C
+hidkhy:u 1)91 c,kfpg q-egiare a, b and c, respectively,
z8kpq *vk uxh z6s(x,3zbnwakl5*c5+ relative to t
b63,8lzzsx h wz*k u+ akqxn vkp(*5c5he origin O.
$\text { Given that } \mathbf{a} \text { is perpendicular to } \overrightarrow{\mathrm{BC}} \text { and } \mathbf{b} \text { is perpendicular to } \overrightarrow{\mathrm{CA}} \text {, prove that } \mathbf{c} \text { is perpendicular to } \overrightarrow{\mathrm{AB}} \text {. }$
