さて、問題です。
4つの辺の長さが全て等しい四角形は?
答えは、ひし形。
本日、休み明けに試験を控える長男(小4)の試験勉強に付き合いました。算数の試験範囲は四角形。
上記の問題に、僕は「正方形」と答えてしまいました。頭が硬い僕には、この答えは目からウロコでした。
例えば、台形。
その定義は、平行な対辺(向かい合う辺)が少なくとも一組ある四角形。ですので、平行四辺形も、ひし形も長方形も正方形も台形なのです。
ちなみに、平行四辺形は、二組の対辺が必ず平行である四角形。ということは、そうです、ひし形や長方形や正方形は、平行四辺形でもあるわけです。
そこで、ひし形ですが、その定義はまさに先の問題のとおり。4 辺の長さが全て等しい四角形。
これに対して、正方形は、4 辺の長さが全て等しく、4 角の大きさが全て直角である四角形。
ですので、正方形はひし形でもあります。先の問題の答えには、もちろん正方形も含まれますが、正方形というだけでは不十分ということです。
だんだん頭が混乱しそうですが・・・
台形といえば、プリンを横から見た形、
平行四辺形といえば、長方形に上から力を加えて右か左に少し押し潰したような形、
すぐにそんなイメージを思い浮かべてしてしまうのは僕だけでしょうか?
思い込みや先入観は、本質から目を逸らさせてしまうことがあるのだと、あらためて気づかされました。
当たり前だと思っていることでも、今一度その本質を考え、確かめるアタマが必要だということです。
小4算数おそるべし。
【岸】
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