P(x,y)到A(-1,-1)B(1,1)距离PA+PB=4的动点P的轨迹
斜椭圆
√[(x+1)^2+(y+1)^2]+√[(x-1)^2+(y-1)^2]=4
(x+1)^2+(y+1)^2=16+(x-1)^2+(y-1)^2+8√[(x-1)^2+(y-1)^2]
4x+4y-16=8√[(x-1)^2+(y-1)^2]
(x+y-4)=2√[(x-1)^2+(y-1)^2]
(x+y-4)^2=4(x-1)^2+4(y-1)^2
x^2+y^2+16+2xy-8x-8y=4x^2-8x+4+4y^2-8y+4
3x^2+3y^2-2xy=12
焦点AB在直线y=x上