标准方程:x2/2+y=1PQ:y=kx+kMN:y=-1/k*(x+1)PQ=2√2(1+k2)/(1+2k2)MN=2√2(k2+1)/(k2+2)S=1/2*PQ*MN=4(1+k2)^2/(1+2k2)(k2+2)对S求导得：k=±1时，S导数=0S最小值为16/9
