∵BM=AM DM=DM BD=DA ∠DMB=∠AMD=90°
∴△dmb全等于△dma,∠dab=∠cba
∵∠dab=2∠cad,∠cad+∠dab+∠cab=90°
∴∠cad+2∠cad+2∠cad=90° ====>>5∠cad=90°====>>∠cad=18°
∴∠CAB=3∠CAd=54°