Let this circle have a radius of r and a diameter of 2r. According to Pythagorean theorem, we can get:

(2r)2=62+82，

4r2=36+64，

4r2= 100，

r2=25，

So r = 5；;

3. 14×52,

=3. 14×25,

=78.5 (square decimeter).

A: The area of this circle is 78.5 square decimeters.