To find the blue area, you have to subtract the area of the small circle from the area of the big circle
The rule of the area of the circle is
[tex]A=\pi(r)^2[/tex]
r is the radius of the circle
Then the area of blue is
[tex]A_{blue}=A_{big}-A_{small}[/tex]
Since the radius of the small circle is 3 units and the radius of the big circle is 7 units, then
[tex]\begin{gathered} A_{blue}=\pi(7)^2-\pi(3)^2 \\ A_{blue}=49\pi-9\pi \\ A_{blue}=40\pi \end{gathered}[/tex]
The exact area of the blue part is a 40pi square unit
