Test and analysis: firstly, do BH⊥AC at point B, and get tan∠ABH=, tan∠ACB=, BH=4x, then AH = 45 3x, then get the value of X according to tan76 =, and finally get the length of BH and AH according to Pythagorean theorem.

Test analysis: if point B is BH⊥AC and the vertical foot is H, then tan∠ABH=,

∵AC=45cm,CD=60cm,AC⊥CD，

∴tan∠CAB=，

∫ AD ∨ BC,

∴∠ACB=∠CAD，

∴tan∠ACB=，

Let BH=4x, then CH=3x, AH = 45-3x,

Tan76 =，

Solution: x=,

∴BH=45,AH=，

∴AB=，

Answer: The length of the crosspiece AB of the car chain is cm.