\mathbfu \cdot \mathbfv = 3(-1) + 4(2) = -3 + 8 = 5 - Parker Core Knowledge

Understanding the Context

The dot product is an operation that takes two vectors and produces a scalar — a single number that encodes how much one vector points in the direction of another. For two 3-dimensional vectors u and v, defined as:

• u = [u₁, u₂, u₃]
  
• v = [v₁, v₂, v₃]

the dot product is defined as:
u · v = u₁v₁ + u₂v₂ + u₃v₃

This formula sums the products of corresponding components of the vectors.

Analyzing u · v = 3(-1) + 4(2) = 5

Key Insights

Let’s interpret the expression 3(-1) + 4(2) step by step:

• The number 3 represents a scalar multiplier associated with the first component (likely the first element of vector u).
  
• (-1) is the first component of vector v.
  
• The number 4 multiplies the second component of v.
  
• (2) is the second component of vector u (or possibly of v, depending on context).

Putting this into vector form:
Suppose:

• u = [-1, 4, ?]
  
• v = [?, ?, 2]

Then:
u · v = (-1)(-1) + (4)(2) = 1 + 8 = 9 — wait, this gives 9, not 5.

To get 5, the expression 3(-1) + 4(2) must correspond to:

• The scalar 3 multiplied by the first component: 3 × (-1)
  
• The scalar 4 multiplied by the second component: 4 × 2

Final Thoughts

Thus, vector u has a first component of -1 and second of 4, while vector v has second component 2 — but the first component is unspecified because it’s multiplied by 3, not directly involved in this evaluation.

This reflects a common teaching method: showing how selective component-wise multiplication contributes to the total dot product.

The Geometric Meaning of the Dot Product

Beyond arithmetic, the dot product is deeply connected to the cosine of the angle θ between two vectors:
u · v = |u||v|cosθ

This means:

• If u · v > 0, the angle is acute (vectors point mostly in the same direction).
  
• If u · v = 0, the vectors are perpendicular.
  
• If u · v < 0, the angle is obtuse.

In our case, u · v = 5, a positive result, indicating that vectors are oriented mostly in the same direction at an acute angle.