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本刊1989年第五期刊出了一篇题为“一道竞赛题的几种解法”的文章。该文对第四届全国中学生物理竞赛预赛第一试的一道选择题作了评价与解答上的分析。提出了两种等效的解答方法,对读者有一定韵启发。但其中第2种等效解法似乎不妥,特提出来商榷。原题及解法(等效法2)摘录于下。原题:如图1所示,P为一块均匀的半圆形薄电阻合金片,先将它按图甲的方式接在电极A、B之间,测得它的电阻为R。然后再将它按图乙的方式接在电极C、D之间,这时的电阻为多少? 等效法2:设半圆形薄电阻合金片半径
In the 1989 fifth issue of the journal, an article entitled “Several solutions to a competition problem” was published. This paper makes an analysis and answer on a multiple-choice question in the first trial of the fourth national junior high school physics competition preliminaries. Two equivalent solutions are proposed, and the reader has certain rhyme inspiration. However, the second equivalent solution does not seem appropriate. The original title and solution (Equivalent 2) are extracted below. The original title: As shown in Figure 1, P is a uniform semi-circular thin resistance alloy piece, which is firstly connected between the electrodes A and B in the way of Fig. A, and its resistance is measured as R. Then, connect it to the electrode C and D according to the method of figure B. What is the resistance at this time? Equivalent method 2: Set the semi-circular thin resistance alloy plate radius