3?0???b
23b?
233?BC的解析式为y??x? ··································································· 2分
4232?y??x?3?x1??1???4（2）由?，得?9
y1??y??3x?3??4??429??0) ?C??1，?，B(2，4???x2?2 ············································· 4分 ??y2?09 ·················································································· 5分 4199?S△ABC??4?? ············································································· 6分
242（3）过点N作NP?MB于点P EO?MB ?NP∥EO
?△BNP∽△BEO ·················································································· 7分 BNNP?? ··························································································· 8分 BEEO?AB?4，CD?由直线y??33?3?x?可得：E?0，? 42?2?53，则BE?
22?在△BEO中，BO?2，EO??62tNP，?NP?t ··········································································· 9分 ?5352216?S?t(4?t)
25312S??t2?t(0?t?4) ········································································ 10分
55312S??(t?2)2? ················································································ 11分
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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