$??解：因為C為AB的黃金分割點，AC \gt BC????$
$所以AC2= AB×BC= AB×(AB-AC)????$
$所以\frac {AC}{AB}=\frac {AB-AC}{AC}=\frac {AB}{AC}-1????$
$設\frac {AC}{AB}=x????$
$則x=\frac {1}{x}-1????$
$所以解得x=\frac {\sqrt{5}-1}{2}或x=\frac {-\sqrt{5}-1}{2}(舍去)$
$????所以\frac {AC}{AB}=\frac {\sqrt{5}-1}{2}$
