x₁, x₂, x₃
     ↓
w₁x₁ + w₂x₂ + w₃x₃ + b
     ↓
     z
     ↓
 sigmoid(z)
     ↓
estimated probability
     ↓
threshold (e.g. 0.5)
     ↓
prediction: yes / no

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