圆锥曲线问题？椭圆x^2/25+y^2/9=1上一点M到焦点F1
椭圆x^2/25 + y^2/9=1上一点M到焦点的距离为2,N是MF1的中点,则/ON/=?(答案是4)?
|MF1| +|MF2| = 2a = 10 ==> |MF2| = 10 -|MF1| = 10-2 = 8 O为F1F2中点，N为MF1中点。ON为MF1的中位线. ==> |ON| = |MF2|/2 = 4