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适度拓展，提升练习效果适度拓展，提升练习效果
六下练习一第5题一共包括3道题目，为更有利于学生比较异同，发现规律，我把3道题目的出示顺序作了如下调整：
（1）小红身高135厘米，小娟身高150厘米。小娟的身高是小红的百分之几？
（2）小红身高135厘米，小娟比小红高15厘米。小娟的身高是小红的百分之几？
（3）小红身高135厘米，小娟身高150厘米。小娟的身高比小红高百分之几？
在学生独立解答（1）和（2）以后，及时组织学生比较这两题的异同。相同点是所求问题相同，都是求“小娟的身高是小红的百分之几？”；不同点是已知条件不同。还有没有不同之处呢？经过适当提示，学生说出两题的解题过程也不同。问题（2）比问题（1）多算一步，必须先算出小娟的身高，再用小娟的身高除以小红的身高。
在比较异同的过程中，让学生进一步体会到已知条件的不同，导致已知条件和要求问题的组合方式不同，使得问题求解的步骤多少不同。在平时的解题练习中，向学生强化审题习惯的教育和培养。
第（3）题照例先让学生独立列式解答。学生因受五下百分数实际问题解题方法的影响，普遍都选择先算小娟比小红高多少厘米，再算小娟的身高比小红高百分之几。这种解法列综合算式是：（150-135）÷135。
为进一步丰富学生的解答方法，完善学生的认知结构，我希望学生能够像掌握第一种解法一样喜欢第二种解法，于是询问学生：第（3）题还有没有其他解法？部分学生会。我根据学生的口答板书：150÷135-100%，并请学生详细解释先算哪部分，求出什么，再算哪部分，求出什么。经过详细的分解，唤起了学生关于此种解法的回忆，知道了“求一个数比另一个数多（或少）百分之几的实际问题”，既可以先算两种数量的差，再用差除以单位“1”的量；也可以先算一个数除以单位“1”的量，再与100%比较：若比100%大，则减去100%，若比100%小，则用100%减去这个百分数。
细心的同学通过观察比较两个综合算式，还发现这两种解法都包括除法和减法两种运算，不过是一种方法先算减法再算除法，另一种方法先算除法再算减法。
经过如此细致的分解，对习题进行适度的拓展，学生不仅理解并掌握了解题的不同方法，还理清了两种不同方法之间的本质联系，使知识由“点状”成为“块状”，更便于纳入已有认知结构，便于学生的后续学习。
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