>>792
ありがとうございます。読んでみようと思います。

>>790
△ABE ∽ △CFE
AB ： BE = CF ： FE = 2 ： 1
CF = 2 * FE
CE^2 = CF^2 + FE^2 = (2 * FE)^2 + FE^2 = 5 * FE^2
BE = CE
AB = 2 * BE = 2 * CE
AE^2 = AB^2 + BE^2 = (2 * CE)^2 + CE^2 = 5 * CE^2 = 25 * FE^2
FE / AE = 1/ 5
AF / AE = (AE + FE) / AE = 1 + FE / AE = 1 + 1/5 = 6/5

↑BC = ↑AC - ↑AB
↑BE = 1/2 * ↑BC
↑AE = ↑AB + ↑BE = ↑AB + 1/2 * ↑BC = ↑AB + 1/2 * (↑AC - ↑AB) = 1/2 * ↑AB + 1/2 * ↑AC
↑AF = 6/5 * ↑AE = 6/5 * (1/2 * ↑AB + 1/2 * ↑AC) = 3/5 * ↑AB + 3/5 * ↑AC