摘要：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? 24．解:(1)点在抛物线上. .··························································································· 2分 解得．··········································································································· 3分 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..．························································ 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．··············································· 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．

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