GH = [-2, -6]
HI = [10, -2]
IJ = [3, 11]
JG = [-11, -3]
∠ H = arccos(-[-2, -6]*[10, -2]/(|[-2, -6]|*|[10, -2]|)) = 82.87°
∠ I = arccos(-[10, -2]*[3, 11]/(|[10, -2]|*|[3, 11]|)) = 93.95°
∠ J = arccos(-[3, 11]*[-11, -3]/(|[3, 11]|*|[-11, -3]|)) = 59.49°
∠ G = arccos(-[-11, -3]*[-2, -6]/(|[-11, -3]|*|[-2, -6]|)) = 123.69°