Md. Asif Uddin
I.2.X01

A second hyperplane

numeric▲△△

For w=(1,3)\vec{w} = (1, 3) and b=6b = -6, write the boundary equation and give the class and signed distance of P=(3,2)P=(3,2), Q=(0,0)Q=(0,0), R=(6,1)R=(6,1), S=(1,0)S=(1,0). Then say which point the boundary would reach first if bb were increased.

Hint

Increasing bb raises every score by the same amount, so it slides the boundary without rotating it. The first point to flip is the one with the smallest positive score.

Solution

Boundary. x1+3x26=0x_1 + 3x_2 - 6 = 0, i.e. x2=2x1/3x_2 = 2 - x_1/3.

Norm. w=1+9=10=3.1623\lVert\vec{w}\rVert = \sqrt{1 + 9} = \sqrt{10} = 3.1623.

PointScoreDistanceClass
P=(3,2)P=(3,2)3+66=+33 + 6 - 6 = +3+0.9487+0.9487+1+1
Q=(0,0)Q=(0,0)0+06=60 + 0 - 6 = -61.8974-1.89741-1
R=(6,1)R=(6,1)6+36=+36 + 3 - 6 = +3+0.9487+0.9487+1+1
S=(1,0)S=(1,0)1+06=51 + 0 - 6 = -51.5811-1.58111-1

Which flips first. Decreasing bb lowers every score equally. PP and RR tie at +3+3, so both flip together at b=9b = -9. Going the other way, increasing bb raises scores: SS at 5-5 flips before QQ at 6-6, at b=1b = -1.

What that shows. The bias is a translation, not a rotation. It moves the boundary along its own normal w\vec{w} and cannot change which side of the plane through the origin a point sits on. Any question that requires re-orienting the boundary needs w\vec{w} to change, and the bias cannot help.

The tie between PP and RR is worth noting: two points at different locations can be equidistant from a boundary, because distance to a hyperplane collapses dd coordinates into one number.

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