①若点O1在AD上，O2在DC上，则AO1=5t，DO1=20－5t，D O2=t 当⊙O1、⊙O2外切，在Rt△O1DO2中
（20－5t）2+（t）2=32，此方程无实数解????????????3分 当⊙O1、⊙O2内切，在Rt△O1DO2中
（20－5t）2+（t）2=12，此方程无实数解???????????4分 ②若点O1，O2在DC上，
17?????????6分 419当⊙O1、⊙O2内切，（5-1）t=19，解得：t??????????8分
41719答：当t=和时，两圆相切．
44当⊙O1、⊙O2外切，（5-1）t=17，解得：t?5)． ·28．解：（1）(0，··········································································································· 1分
1 ······························································································· 2分
P点与C点重合的过程中， （2）在矩形OA1B1C1沿y轴向上平移到
OABC的边BC上时，求得P点移动的距离x?点A1运动到矩形
当0≤x?11． 511时，如图2，由△B2CM1∽△B2A2P， 53?3x113?3x(1?x)． 得CM1?，此时，y?S△B2A2P?S△B2CM1??3?4??42243323452即y??(x?1)?6（或y??x?x?）． ······················································ 4分
8848P点与C点重合时，P点移动的距离x=4． 在矩形OA1B1C1沿y轴向上平移到
11≤x≤4时，如图3，由△B2A2P∽△M1CP， 522216324(4?x)2x?）得CM1?，此时，y?S△PCM??(x?4)（或y?x?． ····· 6分
1333331（3）y?x?4 ??????????????????????????9分
3y 当
B2 C2
A2
