你能估计出这个体育场座位的总数吗？

某体育场看台第1排有a个座位

解：第二排有座位：a 2               a 2=a 2x(2-1)                                                    第三排有座位：a 4               a 4=a 2x(3-1)                             第四排有座位：a 6               a 6=a 2x(4-1)                                             m=a 2(n-1)                                              ...全部

  解：第二排有座位：a 2               a 2=a 2x(2-1)                                                    第三排有座位：a 4               a 4=a 2x(3-1)                             第四排有座位：a 6               a 6=a 2x(4-1)                                             m=a 2(n-1)                                                                                 当a=20    n=12时                                                                                m=20 2x(12-1)                                                         =42   。
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