Solution:
If the outer diameter is 7, then the outer radius is b=7/2.
On the other hand, if the inner diameter is 3, then the inner radius is a= 3/2.
Now, the surface area of a torus (glazed donut) with inner radius a and outer radius b is given by
[tex]SA=\pi^2\mleft(b+a\mright)\mleft(b-a\mright)\text{ =}\pi^2(b^2-a^2)[/tex]Then, applying the data of the problem to the above equation, we can conclude that the surface area of the given glazed donut would be:
[tex]SA=\pi^2(b^2-a^2)=\pi^2((\frac{7}{2})^2-(\frac{3}{2})^2)=98.69[/tex]so that, the correct answer is:
[tex]SA=98.69\approx98.7[/tex]
