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Graphical adding of angles a + b is done: firstly we take on side od the angle and transmit the angle b the outside angle a. This means that the two angles do not overlap if the sum does not exceed 360 °. Their sum is, therefore, an angle, which is defined by their opposite sides.
When subtracting angles a - b we proceed similarly, we only transfer the angle b inwards the angle a. The resulting angle is again outlined by the sides of both angles.
Attention! If a ≤ b, we must pay attention to the orientation od the angle. If you want to avoid these problems, count rather the difference b - a.