Multivariable Calculus

Vectors and Dot Products

01Vectors, Components, and Length in Space 02Scaling and Adding Vectors: The Parallelogram Rule 03The Dot Product and the Law of Cosines 04Finding Angles in Space With the Dot Product 05The Perpendicularity Test and the Normal Vector to a Plane Problem set0/10 Problem set 20/10 MIT problem set0/2

Determinants and the Cross Product

01The Component of a Vector Along Any Unit Direction 02The 2x2 Determinant and the Area of a Parallelogram 03The 3x3 Determinant and the Volume of a Box 04The Cross Product and the Right-Hand Rule 05The Triple Product and the Volume of a Parallelepiped Problem set0/10 Problem set 20/10 MIT problem set0/2

Matrices and Inverse Matrices

01Why Swapping a Cross Product Flips Its Sign 02The Equation of a Plane Through Three Points 03Matrix Multiplication: Rows Dotted With Columns 04Matrices as Transformations: Identity and Rotation 05The Inverse Matrix and Solving AX = B 06Computing a 3x3 Inverse: Minors, Cofactors, Adjoint Problem set0/10 Problem set 20/10 MIT problem set0/2

Planes and Square Systems

01Planes From a Point and a Normal Vector 02Reading the Normal Vector Off a Plane Equation 03Three Planes, One Point: Solving a 3x3 System 04Invertibility, det A, and the Homogeneous System 05When det A = 0: Coplanar Normals and a Line of Solutions Problem set0/10 Problem set 20/10

Parametric Lines and Curves

01Parametric Equations of a Line Through Two Points 02Where a Line Meets a Plane, and When It Never Does 03The Cycloid: Choosing a Parameter That Is Not Time 04Deriving the Cycloid's Equations From Three Vectors 05Is the Cycloid's Bottom a Corner? Taylor Expansion Finds a Cusp Problem set0/10 Problem set 20/10 MIT problem set0/2

Velocity and Kepler's Law

01Velocity and Speed on the Cycloid: Why the Point Stops Dead 02Acceleration Is a Vector: Turning at Constant Speed Still Counts 03Arc Length and the Unit Tangent: v = T̂ ds/dt 04Equal Areas in Equal Times: Kepler's Second Law and |r × v|05Kepler's Second Law Means the Force Is Central Problem set0/10 Problem set 20/10 MIT problem set0/1

Functions of Several Variables

01Functions of Two Variables: Graphs as Surfaces, Built from Slices 02Level Curves and Contour Plots: A Surface Drawn as a Flat Map 03Reading Contour Plots: Domes, Bowls, Saddles and Mountain Passes 04Partial Derivatives: Change One Variable, Hold the Other Fixed Problem set0/10 Problem set 20/10 MIT problem set0/2

Optimization and Least Squares

01Approximation Formula: Changes in x and y Add, via the Tangent Plane 02Critical Points: Where Both Partial Derivatives Are Zero 03Minimum, Maximum or Saddle Point? Classifying by Completing the Square 04Least Squares: The Best-Fit Line as a Minimization in a and b 05Solving Least Squares: Best-Fit Lines, Exponentials and Parabolas Problem set0/10 Problem set 20/10 MIT problem set0/3

The Second Derivative Test

01Quadratic Critical Points: The Sign of 4ac − b² Decides 02The Second Derivative Test: Classifying by the Sign of AC − B² 03Global Min and Max of x + y + 1/(xy): Boundary and Infinity Problem set0/10 Problem set 20/10 MIT problem set0/1

Differentials and the Chain Rule

01The Total Differential: Why df Is Not Δf, and Its Three Uses 02The Chain Rule: Why It Holds, a Worked Check, and the Product Rule 03Chain Rule for a Change of Variables: Partials in Polar Coordinates Problem set0/10 Problem set 20/10 MIT problem set0/1

Gradients and Constraints

01The Gradient Vector: Why It Is Perpendicular to Level Sets 02Tangent Plane to a Surface From the Gradient: The Hyperboloid 03Directional Derivatives and the Direction of Steepest Ascent Problem set0/10 Problem set 20/10 MIT problem set0/2

01Lagrange Multipliers: Tangent Level Curves and ∇f = λ∇g 02Solving the Lagrange Equations, and Why the Method Works 03The Least-Area Gold Pyramid: Its Top Sits Over the Incenter Problem set0/10 Problem set 20/10 MIT problem set0/1

01Implicit Partial Derivatives: Differentiate the Constraint 02Which Variable Is Held Constant? Subscript Notation 03A Constrained Rate Three Ways: Differentials vs Chain Rule Problem set0/10 Problem set 20/10 MIT problem set0/3

Multiple Integrals

01The Heat Equation: A First Partial Differential Equation 02Double Integrals: Volume Under a Surface by Slicing 03Iterated Integrals Over a Square and a Quarter Disk 04Exchanging the Order of Integration: Redraw the Region Problem set0/10 Problem set 20/10 MIT problem set0/1

01Double Integrals in Polar Coordinates: Why dA = r dr dθ 02Area, Mass, Averages and Center of Mass by Double Integrals 03Moment of Inertia: Spinning a Disk About Its Center and Its Rim Problem set0/10 Problem set 20/10 MIT problem set0/3

Area, Mass, Averages and Center of Mass by Double Integrals

How does one double integral give area, mass and averages, and why is a disk's center of mass not found by averaging r and θ?

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Your summary note

1. Area and mass as sums over the region
  Write $\text{Area}(R) = \iint_R 1\,dA$ , define the density $\delta$ as mass per unit area, and build $M = \iint_R \delta\,dA$ from $\Delta m \approx \delta\,\Delta A$ .
2. Uniform and weighted averages of $f$
  Write $\bar{f} = \frac{1}{\text{Area}(R)}\iint_R f\,dA$ and the weighted average $\frac{1}{M}\iint_R f\,\delta\,dA$ , and show it reduces to $\bar{f}$ when $\delta$ is constant.
3. The center of mass $(\bar{x}, \bar{y})$
  Write $\bar{x}$ and $\bar{y}$ as the weighted averages of $x$ and $y$ over the plate, and record the convention $\delta = 1$ when no density is given.
4. Pitfall: averaging $r$ and $\theta$ instead
  Use the disk centered at the origin to show that the averages of $r$ and $\theta$ do not locate its center of mass.
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