拋物線x^2=-16y上一點M到焦點的距離為6,則M的坐標為

拋物線x^2=-16y上一點M到焦點的距離為6,則M的坐標為

2p=16,p=8,p/2=4.
焦點F(0,-P/2),即F(0,-4).
設M(x,y).
FM=√[(x-0)^2+(y+4)^2]=6.
兩邊平方:x^2+y^2+8y+16=36.
-16y+y^2+8y-20=0.
y^2-8y-20=0.
(y-10)(y+2)=0.
y-10=0,y=10 (此點不在拋物線上,捨去.
∴y=-2.
將y=-2代入x^2=-16y中,x^2=32,x=±4√2.
∴M點的坐標為M1(-4√2,-2),M2(4√2,-2).