Problem in Understanding Decision Boundary

In the Decision Boundary video lecture, starting from 6:08, it states that x1 + x2 = 3. But in the graph shown, x1 = 3, and x2 = 3. If that is the case, then x1 + x2 = 6. Similar to the next slide, x12 + x22 = 1. Again, in the graph, x1 = 1 and x2 = 1. Hence, x12 + x22 = 2. Am I missing something here? Or if there anything which I have mis-understood? Thanks.

Hello YC @ycong,

Here is how we read the plot:

The purple straight line goes through the x_2 axis at x_1=0 and x_2=3, and goes through the x_1 axis at x_1=3 and x_2=0. Therefore, both points satisfy the condition of x_1+x_2=3.

The origin has the coordinate of (x_1, x_2) = (0, 0), and along the x_2 axis (or the vertical axis), the coordinates of the ticks are (0, 1), (0,2), and (0,3) respectively. Any point on the x_2 axis has its x_1 value equal to 0.

Raymond