A(0,0),B(2,0),C(3cosT,3sinT)
AB = (2,0),BC = (3cosT-2,3sinT)
AB*BC = 6cosT-4 + 3sinT = 1
6cosT + 3sinT = 5
sin(T + arctan(2)) = 5/根号（45）
T = arcsin(5/根号（45））-arctan(2)
cosT = (10+2根号（5））/15
BC^2 = AB^2 + AC^2 - 2AB*AC cosT = 5-8/根号（5）
BC=根号（ 5- 8/根号（5））