設(shè)拋物線的頂點(diǎn)在原點(diǎn),其焦點(diǎn)F在Y軸上,又拋物線上的點(diǎn)(K,-2)與F點(diǎn)的距離為4,則K等于....A.4 B.4,-4 C.-2 D.2,-2

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解:拋物線是開口方向向下其標(biāo)準(zhǔn)為:x^=-2py,準(zhǔn)線y=p/2由拋物線性質(zhì):(p/2)-(-2)=4,∴p=4∴拋物線方程為:x^=-8y,把點(diǎn)(K,-2)代人x^=-8×(-2)=16x=±4選擇B.4,-4