Prove that the area of 
.
intersects the center of the circle. Connect points
Q and B, since lies on the diameter of the
circle and B lies on the circle, 
is a right angle.
Drop a perpendicular from A to 
. Let the intersection
point be P. Because 
and 
both
intersect chord 
, =. So
is similar to 
by AAA. Therefore,
The base of 
is a so 
.