直角坐标中P(1,0)C：(x-1)^2+(y-1)^2=2（不包括原点）l：y=-x+1联立方程，得到2x^2-2x-1=0直接用两根x1，x2运算PA^2+PB^2=(1+k^2)(x1-xP)^2+(1+k^2)(x2-xP)^2（已知k=-1）=2[x1^2+x^2-2xP(x1+x2)+2xP^2]（xP=1）=2[(x1+x2)^2-2x1x2-2(x1+x2)+2]=4
