If this entire course track has one single most-used piece of math, it's the dot product. It's the operation underneath linear regression (Module 7), the core computation inside every neural network layer (Tier 3), and the way similarity gets measured in search and recommendation systems (Tier 2 and Tier 4). Understanding it well here pays off constantly later.
1-A 2-A 3-A 4-A | 5 6 6 sums/adds 7 = 2*4 + 1*3 = 11 8 = sqrt(36+64) = 10 9 = Neuron outputs, similarities, projections are all dot products | 10 = Cosine similarity of their embedding vectors