r := Abs[fi-Pi/2];
fi1=-Pi/2;
fi2=3Pi/2;
n=300;
points=Table[fi=fi1+(fi2-fi1)*i/n;{r*Cos[fi],r*Sin[fi]},{i,0,n}];
Show[ Graphics[{Line[points],   
 Table[{RGBColor[1,0,0],Polygon[{{0,0},points[[i]],points[[i+1]]}]},{i,n}]
}]];
