又?点B在y??3x?b上 43?0???b
23b?
233···················································································· 2分 ?BC的解析式为y??x? ·
4232?y??x?3?x1??1???4（2）由?，得?9
y1??y??3x?3??4??429???C??1，?，B(2，0)
4???x2?2 ························································· 4分 ?y?0?29 ······································································································· 5分 4199································································································ 6分 ?S△ABC??4?? ·
242（3）过点N作NP?MB于点P ?EO?MB ?NP∥EO
······································································································· 7分 ?△BNP∽△BEO ·
BNNP ···················································································································· 8分 ??BEEO?AB?4，CD?由直线y??33?3?x?可得：E?0，? 42?2?35?在△BEO中，BO?2，EO?，则BE?
2262tNP，?NP?t ······························································································· 9分 ??5352216?S??t?(4?t)
25312·························································································· 10分 S??t2?t(0?t?4) ·
55312······································································································ 11分 S??(t?2)2?
5512?此抛物线开口向下，?当t?2时，S最大?
512····························· 12分 ?当点M运动2秒时，△MNB的面积达到最大，最大为． ·5
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