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Vectors in 2D & 3D Reference cheat sheet - grade 9-12

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Math Grade 9-12

Vectors in 2D & 3D Reference Cheat Sheet

A printable reference covering vector notation, magnitude, direction, components, dot products, and 2D and 3D operations for grades 9-12.

TopicsVectorsAlgebraTrigonometryGeometryMathematicsStudy Guide
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Vectors describe quantities that have both size and direction, such as displacement, velocity, and force. This cheat sheet covers vectors in the coordinate plane and in three-dimensional space. Students need it to quickly remember notation, component form, magnitude formulas, and operations.

It is especially useful for geometry problems involving directed segments, distances, angles, and coordinates.

The most important ideas are that a vector can be written by components, added component by component, and scaled by multiplying each component. Magnitude is found with the distance formula, using x2+y2\sqrt{x^2+y^2} in 22D and x2+y2+z2\sqrt{x^2+y^2+z^2} in 33D. The dot product connects algebra and geometry through u⃗⋅v⃗=∣u⃗∣∣v⃗∣cos⁡θ\vec{u}\cdot\vec{v}=|\vec{u}||\vec{v}|\cos\theta.

Unit vectors help show direction without changing the line of motion.

Key Facts

  • A 22D vector in component form is written v⃗=⟨a,b⟩\vec{v}=\langle a,b\rangle, where aa is the horizontal component and bb is the vertical component.
  • A 33D vector in component form is written v⃗=⟨a,b,c⟩\vec{v}=\langle a,b,c\rangle, where aa, bb, and cc measure movement along the coordinate axes.
  • The magnitude of v⃗=⟨a,b⟩\vec{v}=\langle a,b\rangle is ∣v⃗∣=a2+b2|\vec{v}|=\sqrt{a^2+b^2}.
  • The magnitude of v⃗=⟨a,b,c⟩\vec{v}=\langle a,b,c\rangle is ∣v⃗∣=a2+b2+c2|\vec{v}|=\sqrt{a^2+b^2+c^2}.
  • Vector addition is done by adding corresponding components: ⟨a,b⟩+⟨c,d⟩=⟨a+c,b+d⟩\langle a,b\rangle+\langle c,d\rangle=\langle a+c,b+d\rangle.
  • Scalar multiplication changes the length and possibly the direction: k⟨a,b,c⟩=⟨ka,kb,kc⟩k\langle a,b,c\rangle=\langle ka,kb,kc\rangle.
  • The dot product in 22D is ⟨a,b⟩⋅⟨c,d⟩=ac+bd\langle a,b\rangle\cdot\langle c,d\rangle=ac+bd, and in 33D it is ⟨a,b,c⟩⋅⟨d,e,f⟩=ad+be+cf\langle a,b,c\rangle\cdot\langle d,e,f\rangle=ad+be+cf.
  • The angle between nonzero vectors satisfies cos⁡θ=u⃗⋅v⃗∣u⃗∣∣v⃗∣\cos\theta=\frac{\vec{u}\cdot\vec{v}}{|\vec{u}||\vec{v}|}.

Vocabulary

Vector
A vector is a quantity with both magnitude and direction, often written as v⃗\vec{v} or in component form such as ⟨a,b⟩\langle a,b\rangle.
Magnitude
Magnitude is the length of a vector, found by ∣v⃗∣=a2+b2|\vec{v}|=\sqrt{a^2+b^2} for v⃗=⟨a,b⟩\vec{v}=\langle a,b\rangle.
Component
A component is one coordinate part of a vector, such as aa and bb in v⃗=⟨a,b⟩\vec{v}=\langle a,b\rangle.
Unit vector
A unit vector has magnitude 11 and can be found using v^=v⃗∣v⃗∣\hat{v}=\frac{\vec{v}}{|\vec{v}|} when v⃗≠0⃗\vec{v}\ne\vec{0}.
Dot product
The dot product is a scalar result of multiplying vectors, with u⃗⋅v⃗=∣u⃗∣∣v⃗∣cos⁡θ\vec{u}\cdot\vec{v}=|\vec{u}||\vec{v}|\cos\theta.
Zero vector
The zero vector has all components equal to 00, such as 0⃗=⟨0,0⟩\vec{0}=\langle 0,0\rangle, and has no defined direction.

Common Mistakes to Avoid

  • Adding magnitudes instead of components is wrong because ∣u⃗+v⃗∣|\vec{u}+\vec{v}| is not usually equal to ∣u⃗∣+∣v⃗∣|\vec{u}|+|\vec{v}|.
  • Forgetting the square root in magnitude is wrong because a2+b2a^2+b^2 gives the squared length, while the length is a2+b2\sqrt{a^2+b^2}.
  • Using the dot product as if it were a vector is wrong because u⃗⋅v⃗\vec{u}\cdot\vec{v} produces a scalar, not a component vector.
  • Dividing by a vector to make a unit vector is wrong because vectors are not divided that way; use v^=v⃗∣v⃗∣\hat{v}=\frac{\vec{v}}{|\vec{v}|}.
  • Ignoring direction when using scalar multiplication is wrong because multiplying by a negative scalar reverses the vector direction.

Practice Questions

  1. 1 Find the magnitude of v⃗=⟨6,8⟩\vec{v}=\langle 6,8\rangle.
  2. 2 Given u⃗=⟨3,−2,5⟩\vec{u}=\langle 3,-2,5\rangle and v⃗=⟨−1,4,2⟩\vec{v}=\langle -1,4,2\rangle, find u⃗+v⃗\vec{u}+\vec{v} and u⃗⋅v⃗\vec{u}\cdot\vec{v}.
  3. 3 Find a unit vector in the direction of w⃗=⟨2,−1,2⟩\vec{w}=\langle 2,-1,2\rangle.
  4. 4 Explain how the sign of u⃗⋅v⃗\vec{u}\cdot\vec{v} tells whether the angle between two nonzero vectors is acute, right, or obtuse.