方程的两个根x₁,x₂,x₁ + x₂ = 4
|PM| + |PN| = 4
此为a = 2,c = 1的椭圆,b² = 3
方程:x²/4 + y²/3 = 1c=1是怎么来的？M, N是焦点(-c, 0), (c, 0)