x²+y²=2(cos²t+sin²t)=2曲线C为以(0,0)为圆心,半径为√2的圆。A(1,1),OA的斜率为1,切线l斜率为-1(与x轴的正半轴夹角为3π/4),方程为x+y=2r(cost+sint)=2根号2rsin(t+π/4)=2或根号2rsin(3π/4-t)=2t=0,t0=3π/4则r=根号2/rsin(3π/4-t)