19. 已知空间三点A(−1A(-1A(−1,−1-1−1,2)2)2),B(0B(0B(0,1,0)0)0),C(−2C(-2C(−2,3,1)1)1),设a→=BA→\overrightarrow{a}=\overrightarrow{BA}a=BA,b→=BC→\overrightarrow{b}=\overrightarrow{BC}b=BC,c→=AC→\overrightarrow{c}=\overrightarrow{AC}c=AC.(1)判断ΔABC\Delta ABCΔABC的形状;(2)若(−2a→+kb→)//c→({-2\overrightarrow{a}+k\overrightarrow{b}})//\overrightarrow{c}(−2a+kb)//c,求kkk的值.