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Vectors

An introduction to vectors as arrows and ordered lists, their coordinates, and the two core operations of linear algebra: addition and scalar multiplication.

3Blue1Brown⏱ 10 minOpen on YouTube ↗
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What the lecture covers

The lecture introduces three ways to think about vectors: as arrows with a length and direction, as ordered lists of numbers, and, more abstractly, as objects that can be added and multiplied by numbers. It recommends using arrows rooted at the origin as the main geometric picture, then connecting that picture to coordinates. In two dimensions, a vector’s pair of coordinates tells how far to move along the x- and y-axes; in three dimensions, a third coordinate specifies movement along the z-axis. Each vector corresponds to exactly one coordinate list, and each such list identifies one vector.

Vector addition is defined geometrically by placing the tail of the second vector at the tip of the first; the sum runs from the first vector’s tail to the second’s new tip. This represents taking two movements in succession. In coordinates, corresponding components are added. Multiplication by a scalar stretches or shrinks a vector, and a negative scalar also reverses its direction; numerically, each component is multiplied by that scalar. The lecture emphasizes that linear algebra relies on translating between geometric and numerical views: geometry helps reveal patterns in data, while coordinates let computers represent and manipulate space.

Key ideas

Sample questions

To represent each house using a two-number vector, with one value for square footage and one for price, which interpretation preserves the intended meaning when the order is fixed as stated?

  1. AThe first value is square footage and the second is price.
  2. BThe vector records square footage only; price is not included.
  3. CThe two values can be swapped without changing what the vector represents.
  4. DThe first value is price and the second is square footage.
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Correct answer: A. Each house is represented by an ordered pair, so the position of each value identifies whether it is square footage or price.

When representing a vector as an arrow in a coordinate system, what placement makes its coordinates straightforward to identify?

  1. APlace its head at the origin so the tail’s coordinates represent the vector.
  2. BPlace its tail on the x-axis so its length gives both coordinates.
  3. CPlace it anywhere, because the arrow’s endpoint coordinates alone always represent the vector.
  4. DPlace its tail at the origin so the endpoint’s coordinates represent the vector.
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Correct answer: D. With the arrow starting at the origin, the coordinates of its endpoint give the vector’s coordinate representation.

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