.
設(shè)斜率k
則兩條平行線方程：
y=kx ==> kx-y=0
y-3=k(x-1) ==> kx-y+(3-k)=0
根號5=|3-k|/(k^2+1)^(1/2)
5(k^2+1)=(3-k)^2
2k^2+3k-2=0
(2k-1)(k+2)=0
k=1/2或k=-2
所以,兩條平行線方程：y=(1/2)x 和y=(1/2)x+(5/2)
或：y=-2x 和y=-2x+5