# Operations involving vectors

Vector algebra involves several different types of values; in this course, we study scalars, points, and vectors. We can try to add these, subtract them, and multiply them in various ways. Sometimes these operations make sense, and sometimes they don't, with results as in this table:
Scalar:  Scalar   Scalar   Scalar Vector
Point:  Point  Vector Point
Vector:  Point Vector  Vector  Vector Depends

The operations involving points aren't in the book, but they work just like the operations involving vectors. If P is a point and v is a vector, then P + v is the point obtained by starting at P and moving in the direction and distance given by v. For example, (2, 3) + ⟨2, 1⟩ = (2 + 2, 3 + 1) = (4, 4). Similarly, if P and Q are points, then P − Q is the vector, sometimes denoted P⃗, giving the direction and distance to start at Q and arrive at P. For example, (2, 3) − (4, 1) = ⟨2 − 4, 3 − 1⟩ = ⟨−2, 2⟩. Just as you can write ⟨−2, 2⟩ as −2i + 2j (where i = ⟨1, 0⟩ and j = ⟨0, 1⟩ in 2 dimensions), so you can also write (2, 3) as O + 2i + 3j (where O = (0, 0) in 2 dimensions).

There are various ways to multiply two vectors, with results as in this table:

NameSymbolResult Order matters?Depends on lengths? Depends on orientation?
Dot product ScalarNo YesNo
Cross product× Scalar in 2D,
vector in 3D
YesYes Yes
There are actually other ways to multiply vectors besides these, but we don't cover them in this course.

The cross product in 2 dimensions is not in the textbook; here is the formula for it:

a, b⟩ × ⟨c, d⟩ = ad − bc.
For example, ⟨−2, 2⟩ × ⟨3, 1⟩ = (−2)(1) − (2)(3) = −8. Geometrically,
u × v = |u| |v| sin∠(u, v),
where ∠(u, v), the measure of the angle from u to v, is positive if this angle is counterclockwise (using a counterclockwise coordinate system, which is like using a right-handed coordinate system in 3 dimensions) and negative if it's clockwise.

Recall that subtraction is adding the opposite: a − b = a + (−b), and u − v = u + (−v). Similarly, the cross product in 2 dimensions (but not in 3 dimensions) can be done using the dot product and a rotation: u × v = u ⋅ (×v), where ×v is obtained from v by rotating it clockwise through a right angle (using a counterclockwise coordinate system again). The formula for this is

×⟨c, d⟩ = ⟨d, −c⟩.
For example, ×⟨3, 1⟩ = ⟨1, −3⟩, so ⟨−2, 2⟩ × ⟨3, 1⟩ = ⟨−2, 2⟩ ⋅ ⟨1, −3⟩ = (−2)(1) + (2)(−3) = −8.
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This web page was written between 2014 and 2017 by Toby Bartels, last edited on 2017 March 18. Toby reserves no legal rights to it.

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