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易错点一应用两个基本原理时或犯“遗漏”与“重复”的错误,或分不清应该用加法原理还是乘法原理例1有红、黄、蓝旗各3面,每次升一面、二面、三面在某一旗杆上纵向排列,表示不同的信号,顺序不同则表示不同的信号,共可以组成多少种不同的信号?错解可组成3×3×3=27种不同的信号.正解每次升1面旗可组成3种不同的信号;每次用2面旗可组成3×3=9种不同的信号;每次升3面旗可组成3×3×3=27种不同的信号.根据分类计数原
Error-prone point: When applying two basic principles, it either commits “missing” and “repeat” errors, or can’t tell whether it should use the principle of addition or multiplication principle. Example 1 has three sides of red, yellow and blue flags. Each time, one side, two sides, and three sides are arranged vertically on a certain flagpole to indicate different signals. Different orders indicate different signals. How many different signals can be formed? The wrong solution can be composed of 3×3×3=27. Different kinds of signals. Positive solution Each time a flag is raised, 3 different signals can be formed; each time 2 flags can be used to form 3 × 3 = 9 different signals; each time 3 flags are raised, 3 × 3 × can be formed. 3=27 different signals. Count original according to classification