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GATE DA 2025 Q53: Hard-margin SVM on four points in ℝ²

Question

Consider designing a linear binary classifier f(x)=sign⁡(w⊤x+b)f(x) = \operatorname{sign}(w^{\top}x + b), x∈R2x \in \mathbb{R}^2 on the following training data:

Class-1: {(20),(02),(22)},Class-2: {(00)}\text{Class-1: } \left\{ \begin{pmatrix} 2 \\ 0 \end{pmatrix}, \begin{pmatrix} 0 \\ 2 \end{pmatrix}, \begin{pmatrix} 2 \\ 2 \end{pmatrix} \right\}, \quad \text{Class-2: } \left\{ \begin{pmatrix} 0 \\ 0 \end{pmatrix} \right\}

Hard-margin support vector machine (SVM) formulation is solved to obtain ww and bb. Which of the following options is/are correct?

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Answer: B, C (official GATE 2025 final answer key)

Concept used: Maximum-margin hyperplane, support vectors and margin 2/∥w∥2/\lVert w \rVert.

First step: Class-1's closest points to the origin, (2, 0) and (0, 2), lie on the line x1+x2=2x_1 + x_2 = 2, so the best boundary is parallel to it, halfway to the origin: x1+x2=1x_1 + x_2 = 1.

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Source: official GATE 2025 DA question paper and final answer key, organised by IIT Roorkee. Question paper · Answer key