在平面直角坐标系内，点P（x，y）的横坐标、纵坐标都在{0，1，2，3}内取值，
（1）不同的点P共有多少个？
（2）在上述点中，不在坐标轴上的点有多少个？
解：（1）可以分两步确定点P的坐标，第1步，确定横坐标，有4种可能；
第2步，确定纵坐标，有4种可能。
根据分步乘法计数原理，不同的点P共有16个。
（2）当P不在坐标轴上时，横、纵坐标都在{1，2，3}内取值，
类似（1）的解法，共有3×3=9（个）。
| 完形填空。 | |||
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