We have,
Radius of outer semi circle r1 = 5 unit
        Area   $=\frac{\mathrm{\pi }}{2}{{\mathrm{r}}_1}^2=\frac{\mathrm{\pi }(5{)}^2}{2}$ s.q. unit
 Radius of inner semi circle r2 = 5 – 1 = 4 unit
 Area  $=\frac{\mathrm{\pi }}{2}{\mathrm{r}}_2^2=\frac{\mathrm{\pi }}{2}(4{)}^2$sq. unit
 then, the area of the shaded portion$=\frac{1}{2}\left({\mathrm{\pi r}}_1^2-{\mathrm{\pi r}}_2^2\right)$
$=\frac{\mathrm{\pi }}{2}(5{)}^2-\mathrm{\pi }(4{)}^2=\frac{\mathrm{\pi }}{2}\left(5^2-4^2\right)$
$=\frac{\mathrm{\pi }}{2}(25-16)=4.5\mathrm{\pi }$   sq unit