In diagram 1, identical transmitters are positioned at the vertices of an equilateral triangle. The equilateral triangle has sides of length 20km and area 173kmsquared. The transmitters are placed at A, B and C, and they emit a frequency in a circular radius up to 10km. The unshaded area inside the triangle represents the part of the triangle that receives no signal. Find the area of the triangle that does not receive a signal