A(X1,Y1)B(X2,Y2)C(X3,Y3)求向量AB,BC,CA ,并验证向量AB+BC+CA=0
是不是这样的,AB=-BC-CA=(X2-X1,Y2-Y1)                        BC=-CA-AB=(X3-X2,Y3-Y2)                        CA=-AB-BC=(X1-X3,Y1-Y3)接着是不是AB+BC+CA=(X2-X1,Y2-Y1)+(X3-X2,Y3-Y2)+(X1-X3,Y1-Y3)                    =(X2-X1+X3-X2+X1-X3,Y2-Y1+Y3-Y2+Y1-Y3)                    =(0,0)                  是不是这样的?