以下是Python实现的三边定位算法的一部分，想问一下当“idx等于0时，计算的是目标点到第4个信标的距离。当idx不等于0时，计算的是目标点到第1个信标的距离”如此计算的理由，还有这部分的数学表达式是什么？
self.position = np.array([[-0.5, -0.5],
[5.5, -0.5],
[5.5, 5.5],
[-0.5, 5.5]])
self.distances = []
self.result = 0
def trilaterate2D(self):
A = []
B = []
# trilateration using SVD
for idx in range(4):
if idx == 0: # i:1 j:4
x_coefficient = self.position[3][0] - self.position[idx][0] # x1-xidx
y_coefficient = self.position[3][1] - self.position[idx][1] # y1-yidx
b = 1 / 2 * (self.distances[idx] ** 2 - self.distances[3] ** 2 -
((self.position[idx][0] - self.position[3][0]) ** 2 + (
self.position[idx][1] - self.position[3][1]) ** 2)) \
+ x_coefficient * self.position[3][0] + y_coefficient * self.position[3][1]
A.append([x_coefficient, y_coefficient])
B.append([b])
else:
x_coefficient = self.position[0][0] - self.position[idx][0] # x1-xidx
y_coefficient = self.position[0][1] - self.position[idx][1] # y1-yidx
b = 1 / 2 * (self.distances[idx] ** 2 - self.distances[0] ** 2 -
((self.position[idx][0] - self.position[0][0]) ** 2 + (
self.position[idx][1] - self.position[0][1]) ** 2)) \
+ x_coefficient * self.position[0][0] + y_coefficient * self.position[0][1]
A.append([x_coefficient, y_coefficient])
B.append([b])
B = np.array(B)
A_pseudo = np.linalg.pinv(A)
self.result = np.dot(A_pseudo, B)
result_x = self.result[0]
result_y = self.result[1]
# return x, y position
return result_x, result_y