英 | 日 |
---|---|
To Obtain a 30-degree Angle | 30°角を折り出す |
1. Fold and unfold to crease in the middle. | 1. 真ん中に縦の折り目を付ける。 |
2. Crease from the lower right corner so that the lower left corner is on the middle vertical crease. | 2. 左下角を縦の折り目に合わせて右下角から折り目を付ける。 |
3. Unfold. Note that a 30-degree slope has been created. | 3. 戻すと 30°角ができている。 |
Proof: | 証明: |
AC = BC because AC came from BC. AC = AB because △ABC is an isosceles triangle. Thus AB = AC = BC, which means △ABC is an equilateral triangle. ∠ACB = 60° ∠DCB is a half of ∠ACB Thus ∠DCB = 30° |
AC は元々 BC から来たので AC = BC △ABC は二等辺三角形なので AC = AB ゆえに AB = AC = BC すなわち △ABC は正三角形である。 ゆえに ∠ACB = 60° ∠DCB は ∠ACB の半分なので ∠DCB = 30° |
(Both English and Japanese parts have been authored by Osamu Takeuchi.)