The formula for the area of a circle is:
[tex]A=\pi\cdot r^2[/tex]also the diameter can be written in function of the radius
[tex]D=2\cdot r[/tex]from this equation we can find the radius
[tex]r=\frac{D}{2}[/tex]this transforms the formula for the area into
[tex]\begin{gathered} A=\pi\cdot(\frac{D}{2})^2 \\ A=\frac{\pi}{4}\cdot D^2 \end{gathered}[/tex]since the diameter is equal to 26cm, then the area will be
[tex]\begin{gathered} A=\frac{\pi}{4}\cdot676cm^2 \\ A=169\pi(cm^2)\approx530.93cm^2 \end{gathered}[/tex]