what is the area of a semi circle id the radius is 7.89cm

The area of a full circle is pi*radius², so the area of a semicircle is (1/2)pi*radius².

$$\left({\frac{{\mathtt{1}}}{{\mathtt{2}}}}\right){\mathtt{\,\times\,}}{\mathtt{\pi}}{\mathtt{\,\times\,}}{{\mathtt{7.89}}}^{{\mathtt{2}}} = {\mathtt{97.785\: \!370\: \!015\: \!268\: \!583\: \!8}}$$

area ≈ 97.79 cm²