已知平面直角坐标系中,点O为原点,A(-3,-4),B(5,-12),求OA*OB?

题目

已知平面直角坐标系中,点O为原点,A(-3,-4),B(5,-12),求OA*OB?
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答案

O(0,0),A(-3,-4),B(5,-12),
OA^2=(0+3)^2+(0+4)^2=25,OA=5;
OB^2=(0-5)^2+(0+12)^2=169,OB=13;
OA*OB=5*13=65.