∴p=2.故选C.
答案:C
5.焦点在x轴的负半轴上,并且过点(-4,2)的抛物线的标准方程为 .
解析:设所求抛物线的标准方程为y2=-2px(p>0).
因为抛物线过点(-4,2),
所以22=-2p×(-4),
即p=1/2.
故所求抛物线的标准方程为y2=-x.
答案:y2=-x
6.若抛物线y2=4x上一点到焦点的距离为5,则这点的坐标为 .
答案:(4,4)或(4,-4)
7.设抛物线y2=2px(p>0)的焦点为F,已知点A(0,2).若线段FA的中点B在抛物线上,则点B到该抛物线准线的距离为 .
解析:由已知,得F(p/2 "," 0),∴B(p/4 "," 1),
∴2p×p/4=1,解得p=√2.
∴B(√2/4 "," 1).
因此点B到该抛物线的准线的距离为 √2/4+√2/2=(3√2)/4.
答案:(3√2)/4
8.已知抛物线的顶点在原点,对称轴是x轴,抛物线上的点M(-3,m)到焦点F的距离等于5,求抛物线的方程和m的值.
分析:由题意可先设抛物线方程为y2=-2px(p>0),再求解.
解:设抛物线方程为y2=-2px(p>0),
则焦点F("-" p/2 "," 0),
由题意可得{■(m^2=6p"," @√(m^2+(3"-" p/2)^2 )=5"," )┤
解得{■(m=2√6 "," @p=4)┤或{■(m="-" 2√6 "," @p=4"." )┤