∵PQ∥RS,BN⊥RS,CM⊥PQ
∴BN∥CM
∴∠MCB=∠CBN
又∵BN,CM为角平分线
∴∠DCM=∠BCM,∠CBN=∠ABN
∴2∠MCB=2∠CBN 即 ∠DCB=∠CBA
∴DC∥AB