ファイ
覚え方のコツ Memory Tip
phi はギリシャ文字の第21字 Φ・φ の英語名で、英語では一般に「ファイ」と発音します。数学では黄金比、角度、関数など分野ごとに異なる量を表す記号なので、常に黄金比という意味ではありません。形や綴りが似て見える pi「パイ」π と区別し、golden ratio φ ≈ 1.618 の代表例で覚えましょう。
Remember phi as the name of the Greek letter Φ or φ, not as a word with only one technical meaning. In English it is commonly pronounced “fie.” Its best-known mathematical use is the golden ratio, approximately 1.618, but different fields also use phi for angles, functions, phases, and other quantities. Keep it separate from pi, the Greek letter π: pi often represents the circle constant 3.14159…, whereas phi is a different letter and its meaning depends on context.
phi の語源・成り立ち Etymology
ギリシャ文字の第21字。ギリシャ語 φεῖ に由来し、元はフェニキア文字の「口」を意味する pe が起源。数学では黄金比（φ≈1.618）を表し、彫刻家フェイディアス（Pheidias）の頭文字から名付けられた。関連語：philosophy（哲学、φίλος＋σοφία）。
Phi is the English name of the twenty-first letter of the Greek alphabet, Φ or φ. Its name comes through Latin from Greek pheî. The Greek letter belongs to an alphabet whose earlier history is connected with Phoenician writing. In modern mathematics and science, phi is assigned several conventional symbolic roles. Its use for the golden ratio is one such convention, not the original lexical meaning of the letter’s name.
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