∵∠BAP+∠BPA=90°,∠BPA+∠CPQ=90°,
∴∠BAP=∠CPQ,又∠B=∠C=90°,
∴△ABP∽△PCQ,
∴ = ,
∴CQ= = =- x2+x,
∴DQ= x2-x+4
∴S△ADQ= AD?DQ= ×4( x2-x+4)
= x2-2x+8,
∴當x=- =2時,S△ADQ=6.