2009 AMC 8 第 23 题

先试着解答 2009 AMC 8 第 23 题,然后核对你的答案与精心整理的解答,解答来自 LIVE by Po-Shen Loh。你也可以参加完整限时模拟考试、查看全部 2009 AMC 8 解答,或核对答案。

所有题目均经美国数学协会(MAA)官方合法授权使用。

23.

在学校最后一天,旺德福老师给班上的学生发果冻豆。她给每个男生的果冻豆数等于班上男生人数;给每个女生的果冻豆数等于班上女生人数。她带了 400400 颗果冻豆,发完后剩下六颗。班上男生比女生多两人。她班上有多少名学生?
On the last day of school, Mrs. Wonderful gave jelly beans to her class. She gave each boy as many jelly beans as there were boys in the class. She gave each girl as many jelly beans as there were girls in the class. She brought 400400 jelly beans, and when she finished, she had six jelly beans left. There were two more boys than girls in her class. How many students were in her class?
2626
2828
3030
3232
3434
答案:B
知识点:二次方程因式分解
小提示:
设女生人数为 gg,则男生人数为 g+2g+2
Let the number of girls be g,g, so the number of boys is g+2.g+2.
大提示:
发出的果冻豆总数为 400−6400-6
The total jelly beans given out is 400−6.400-6.
解答:
设班上男生人数为 bb,女生人数为 gg。由题意,b=g+2b = g + 2。
如果每个男生得到 bb 颗果冻豆,那么给所有男生共发出 b2b^2 颗。同样,给所有女生共发出 g2g^2 颗。
因此 b2+g2=400−6(g+2)2+g2=3942g2+4g+4=394g2+2g−195=0(g+15)(g−13)=0。 \begin{gather*} b^2 + g^2 = 400 - 6 \\ (g + 2)^2 + g^2 = 394 \\ 2g^2 + 4g + 4 = 394 \\ g^2 + 2g - 195 = 0 \\ (g + 15)(g - 13) = 0 \end{gather*}\text{。}因为 gg 不能为负,所以 g=13g = 13。这意味着 b=15b = 15,所以 b+g=28b + g = 28。
所以正确答案是 B。
Let bb be the number of boys in the class and gg be the number of girls. From the problem, we get that b=g+2.b = g + 2.
If each boy gets bb jelly beans, then Mrs. Wonderful will give out a total of b2b^2 jelly beans to all the boys. Similarly, she will give out g2g^2 jelly beans to all the girls.
Therefore, b2+g2=400−6(g+2)2+g2=3942g2+4g+4=394g2+2g−195=0(g+15)(g−13)=0. \begin{gather*} b^2 + g^2 = 400 - 6 \\ (g + 2)^2 + g^2 = 394 \\ 2g^2 + 4g + 4 = 394 \\ g^2 + 2g - 195 = 0 \\ (g + 15)(g - 13) = 0. \end{gather*} Since gg cannot be negative, we get that g=13.g = 13. This means that b=15,b = 15, so b+g=28.b + g = 28.
Thus, B is the correct answer.
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