设x秒.则AP=x,BQ=2x
PD^2=144+x^2,pq^2=(6-x)^2+4x^2,dq^2=36+(12-2x)^2
pd^2=pq^2+dq^2
144+x^2=(6-x)^2+4x^2+36+(12-2x)^2
x=3或6