平行四邊形ABCD中，A (2,-4) B (X1,Y1) C (3,-1) D (X2,Y2)，求X1+X2+Y1+Y2的算式?

因為ABCD是平行四邊形, 所以 :
AB向量 = DC向量
( X1 - 2 , Y1 + 4 ) = ( 3 - X2 , -1 - Y2 )
X1 - 2 = 3 - X2 且 Y1 + 4 = -1 - Y2
X1 + X2 = 5 且 Y1 + Y2 = - 5
X1 + X2 + Y1 + Y2 = 5 + (-5) = 0

Ans: 0

B(2.-1) D(3.-4)
2+(-1)+3+(-4)=0