AMC 8 1988 Problem 9
AMC8 1988 年第 9 题
三角形★★☆☆☆
An isosceles triangle is a triangle with two sides of equal length. How many of the five triangles on the square grid below are isosceles?
等腰三角形是有两条边长度相等的三角形。下面方格网格上的五个三角形中有多少个是等腰三角形？
- A.1
- B.2
- C.3
- D.4
- E.5
正确答案 · Correct Answer
D
解析 · Solution
Solution 1
Going from top left to bottom right, the first triangle has two sides of length by the Pythagorean theorem, so it is isosceles.
The second triangle has two sides of length 2, so it is isoceles.
The third triangle has sides of length 2, , and , so it is not isosceles.
The fourth triangle has two sides of length so it is isosceles.
The fifth triangle has two sides of length , so it is isosceles.
So, 4 of these triangles are isosceles.
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