全国中考数压轴题精选精析.doc

《全国中考数压轴题精选精析.doc》由会员上传分享，免费在线阅读，更多相关内容在行业资料-天天文库。

1、2008年全国中考数学压轴题精选精析25.（08江西南昌）24．如图，抛物线相交于两点．（1）求值；（2）设与轴分别交于两点（点在点的左边），与轴分别交于两点（点在点的左边），观察四点的坐标，写出一条正确的结论，并通过计算说明；（3）设两点的横坐标分别记为，若在轴上有一动点，且，过作一条垂直于轴的直线，与两条抛物线分别交于C，D两点，试问当为何值时，线段CD有最大值？其最大值为多少？（08江西南昌24题解析）24．解：（1）点在抛物线上，，·································

2、··························································2分解得．···········································································································3分（2）由（1）知，抛物线，．·······5分当时，解得，．点在点的左边，，．············6分当时，解得，．点在点的左边，，．··············

3、··········································7分，，点与点对称，点与点对称．·······························································8分（3）．抛物线开口向下，抛物线开口向上．················9分根据题意，得．···············································11分，当时，有最大值．····························

4、···················12分说明：第（2）问中，结论写成“，四点横坐标的代数和为0”或“”均得1分．26.（08江西南昌）25．如图1，正方形和正三角形的边长都为1，点分别在线段上滑动，设点到的距离为，到的距离为，记为（当点分别与重合时，记）．（1）当时（如图2所示），求的值（结果保留根号）；（2）当为何值时，点落在对角形上？请说出你的理由，并求出此时的值（结果保留根号）；（3）请你补充完成下表（精确到0.01）：0.0300.290.290.130.03（4）若将“点分别在线段上滑动”改

5、为“点分别在正方形边上滑动”．当滑动一周时，请使用（3）的结果，在图4中描出部分点后，勾画出点运动所形成的大致图形．（参考数据：．）（08江西南昌25题解析）25．解：（1）过作于交于，于．，，，．······················································································2分，．··································································

6、·······················3分（2）当时，点在对角线上，其理由是：··········································4分过作交于，过作交于．平分，，．，，．，．，．即时，点落在对角线上．································································6分（以下给出两种求的解法）方法一：，．在中，，．··············································

7、····························7分．························································································8分方法二：当点在对角线上时，有，···························································································7分解得．·······················

8、·································································8分（4）由点所得到的大致图形如图所示：12分说明：1．第（2）问回答正确的得1分，证明正确的得2分，求出的值各得1分；2．第（3）问表格数据，每填对其中4空得1分；3．第（4）问图形画得大致正确的得2分，只画出图形一部分的得1分．27.（08山东滨州）23、（1）探究新知：如图1，已知△ABC与△