2025 AMC 8 第 24 题
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24.
在梯形 中,角 和角 都是 ,且 。所有边长都是正整数,梯形 的周长为 个单位。有多少个互不全等的梯形满足这些条件?
In trapezoid angles and measure and The side lengths are all positive integers and the perimeter of is units. How many non-congruent trapezoids satisfy all of these conditions?
答案:E
视频讲解:
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文字解答:
过 作一条平行于 的线,并观察到有些长度会自动相等:
这是因为 ,所以三角形 是等边三角形。所有标为 的线段总是相等。
另外, 是平行四边形(不一定是菱形),所以其余标为 的线段彼此相等,但不一定等于 。
周长为 ,题目要求它等于 。
问题就变成数方程 有多少组正整数解。
考虑正整数 时,只有偶数值会给出整数 ,因为 必须是偶数 。
而当 时,满足方程的 为 ,不是正整数。
所以可行的 是小于 的正偶数,即 。共有 种,答案是 E。
Draw a line through parallel to line and observe that some lengths are automatically equal to each other:
That is because and so triangle is equilateral. All lengths labeled are always equal.
Also, is a parallelogram (not necessarily a rhombus), so the remaining lengths labeled are always equal to each other, but not necessarily equal to
The perimeter is but it is also supposed to be
The problem then amounts to counting how many positive integer solutions there are to the equation
As we consider positive integers for observe that it is precisely the even integers which give an integer solution for because is the even number
And, once we get to the that satisfies the equation is which is not a positive integer.
So, the values that work for are the positive even integers less than or There are options, which gives the answer of E.
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