(1)lAC: x-y+1=0
2x-3y+1=0x=-2 y=-1
lAC:x-y+1=0
lAB:3x+2y+c=0
把A(1,2)代入 c=-7
lAB:3x+2y-7=0
点B的坐标 x+y=0
3x+2y-7=0x=7 y=-7
B=(7,-7)
点C的坐标 x-y+1=0
2x-3y+1=0x=-2 y=-1
c(-2,-1)
lBC: (x+2)/(-9)=(y+1)/62X+3Y+7=0