Simple. Because AB: AP = AP: Pb, P is the golden section of line segment AB. According to the definition of golden section, we can see AB: AP = AP: Pb = 0.6 18. Because PB= 1, AP: Pb = AP: 1 = 0.6 18, it is concluded that AP=0.6 18. Because AB: So AB=(0.6 18)*2=3.72.