过圆心O作OH⊥AB于H,则AH=DH,BH=CH,AB*BD=(AH-BH)(DH+BH)=(AH+BH)(AH+BH)=AH^2-BH^2=(AO^2-OH^2)-(BO^2-OH^2)=AO^2-BO^2=7^2-5^2=24.
A在哪？
