设直线方程为y-2=k(x-1),k<0
与坐标轴交点为((k-2)/k,0),(0,2-k)
S=(2-k)*(k-2)/k/2=-(k-2)^2/(2k)
S'=-2k/(2k)+(k-2)^2/(2k^2)=0
k=-2±2√2
因k<0
取k=-2-2√2
直线为y-2=(-2-2√2)(x-1)
y=-2(1+√2)x+4+2√2