It's a equilateral triangle. The area of the white region is the area of a sector minus the area of a triangle.
 
 The sector is 1/6 of the circle, therefore the area of the sector is 1/6 of area of the circle.
 The formula of an area of the circle: 
 
  The area of the sector: 

 The formula of an area of the equilateral triangle: 
 
  
 The area of the segment: 

 The area of the shaded region: 
