let's keep in mind that, the large circle has a radius of AB, whilst the small circle has a radius of AB/2.
[tex]\bf \textit{area of the large circle}\\\\
A=\pi r^2~~
\begin{cases}
r=radius\\
-----\\
r=AB
\end{cases}\implies A=(AB)^2\pi \\\\
-------------------------------\\\\
\textit{area of the small circle}\\\\
A=\pi r^2~~
\begin{cases}
r=radius\\
-----\\
r=\frac{AB}{2}
\end{cases}\implies A=\left( \frac{AB}{2} \right)^2\pi\implies A=\cfrac{(AB)^2}{2^2}\pi \\\\
-------------------------------[/tex]
[tex]\bf \textit{now subtracting the small area from the large one}\\\\
\textit{what's leftover is the \underline{shaded area}}
\\\\\\
\stackrel{\textit{large's area}}{(AB)^2\pi }~~-~~\stackrel{\textit{small's area}}{\cfrac{(AB)^2\pi }{4}}~~\stackrel{LCD~of~4}{\implies }~~\cfrac{4(AB)^2\pi -(AB)^2\pi }{4}
\\\\\\
\cfrac{3(AB)^2\pi }{4}\implies \cfrac{3}{4}(AB)^2\pi \impliedby \frac{3}{4}\textit{ of the large's area}[/tex]
