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  • Grasping Related Rates Within Calculus

  • The Importance of Related Rates

  • Key Formulas in Related Rates

  • Methodical Approach to Solving Problems on Related Rates

  • Demonstration of Related Rates Problems with Solutions

  • Core Insights on Related Rates

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Related Rates: Explainations, Formula & Calculation Examples

Casting a pebble into still waters leads to the formation of expanding concentric ripples. These ripples, embodying waves with a continuously altering radius and area, illustrate change. Related rates in the realm of Calculus explore how to calculate the variation in one quantity by connecting it to others whose rates of alteration are known.

Consider the scenario where the radius of a wave expands, affecting its contained area. Understanding the alteration rate of one quantity enables the prediction of another's change rate. This conceptualization stems from the relationship between a circle's area, \(A=\pi r^2\), and its radius, \(r\).

Addressing related rates problems necessitates competencies in Implicit Differentiation and The Chain Rule, foundational elements of Calculus. A prerequisite review of these concepts is pivotal before engaging with related rates challenges. Proficiency in related rates not only deepens your calculus insights but also finds relevance across disciplines such as finance, physics, and engineering, making the complex simpler through rate of change analyses.

Within Calculus, problems concerning related rates are primarily grouped under:

  • Area or volume calculation
  • Applications in trigonometry

Familiarity with previously learned volume/area and trigonometric principles is essential. Presented below are key formulas pivotal for solving related rates scenarios.

Surface Area Calculations

For Triangles

  • \(area=\dfrac{1}{2}b \cdot h\) with \(b\) representing the base and \(h\), the height

Rectangles or Squares

  • \(area=h \cot w\), \(h\) and \(w\) denote height and width, respectively.


  • \(area=\pi r^2\), \(r\) being the radius.
  • \(circumference = 2 \pi \cdot r\)

Volume Measurements

Pyramidal Structures

  • \(volume=\dfrac{1}{3} L \cdot W \cdot H\), where \(L\), \(W\), and \(H\) refer to length, width, and height respectively


  • \(volume= \dfrac{1}{3} \pi \cdot r^2 \cdot h\), here, \(r\) and \(h\) indicate radius and height

Cylindrical Shapes

  • \(volume= \pi \cdot r^2 \cdot h\), with \(r\) and \(h\) symbolizing radius and height

Solid Rectangular Forms

  • \(volume= L \cdot W \cdot H\) identifying \(L\), \(W\), and \(H\) as length, width, and height


  • \(volume=\dfrac{4}{3} \cdot \pi r^3\) where \(r\) represents the radius

Trigonometric Relations

Refer to the diagram below for symbols explanation.

Trigonometric Ratios (SOH-CAH-TOA)




The Theorem of Pythagoras

  • \(c^2=a^2+b^2\)

Addressing related rates issues effectively entails a structured method. Below is a universally applicable strategy for resolving rate of change dilemmas:

Step 1: Illustrate Through Drawing

Initiate by sketching a diagram capturing the given information along with symbols for sought quantities.

Step 2: Discern Knowns from Unknowns

Examine the problem to deduce given data versus sought information. Employ this intel to annotate your sketch.

Step 3: Relating Variables Through Equations

Forge a relationship between known and unknown variables via an equation, a critical stride in tackling related rates predicaments.

Step 4: Derive Using Implicit Differentiation

Implement implicit differentiation on your relational equation to unearth variable rates of change.

Step 5: Inserting Known Metrics

Integrate provided metrics into your equation to resolve for the desired rate of change.

Upon resolution, interpreting the result within the problem's framework is crucial, focusing on the significance and directionality of the outcome.

In case of related rates queries in AP Calculus contexts, differentiate with temporal consideration when applying implicit differentiation.

Delving into related rates through a practical example involving a ladder and a wall.

Illustrative Example

A ladder of \(10ft\) in height is propped against a wall. It starts slipping away from the wall at a velocity of \(2ft/s\). As it maneuvers away, the ladder's apex slides down the wall. Given the ladder's base distance from the wall is \(9ft\), at what velocity does the top proceed downwards?

Step 1: Depict Through Diagrams

Sketching aides in visualizing what is known and what needs discovery.

Within our sketch, the vertical rate seeks calculation while the horizontal speed and ladder length are provided.

Step 2: Sorting Knowns from Unknowns

Prior to Calculus application, comprehensive problem understanding is pivotal. We're informed of a \(10ft\) ladder's horizontal receding velocity of \(2ft/s\), with the question focusing on its top's descent rate upon the base distancing \(9ft\) from the wall.

Labelling in our step 1 diagram aids in organizing variable data:






Here, \(x\) and \(y\) are depicted as temporal functions, hence \(x(t)\) and \(y(t)\), contrastingly, \(z\)'s constancy negates time-relation in its notation.

Step 3: Equation Synthesis for Relating Data

The puzzle pieces - our knowns versus our quest - suggest Pythagorean Theorem's applicability.

Observing the diagram, the ladder alongside the walls crafts a perfect right triangle, a Pythagorean theorem exemplary scenario.

While the ladder inclines and declines, its triangular hypotenuse, the ladder itself, remains constant.

\[(x(t))^2+ (y(t))^2=z^2\]

\[(x(t))^2+ (y(t))^2=10^2\]

\[(x(t))^2+ (y(t))^2=100\]

Given, we're furnished with \(x\)'s time differential,


The inquiry pertains to the vertical movement's rate,


What strategy enables formulating an equation incorporating these variables? Implicit differentiation!

Step 4: Implicit Differentiation Application

With an equation in place, we proceed with implicit differentiation, delineating the equation in terms of variable change rates over time.

\[\dfrac{d}{dt}[(x(t))^2+(y(t))^2]=\dfrac{d}{dt} 100\]


Step 5: Inferring from Known Information

The ultimate quest - determining how swiftly the ladder descends:


With \(x=9\) ft and


Integrating our knowns yields

\[2 \cdot 9 \cdot 2 + 2(y(t))\dfrac{dy}{dt}=0\]

For \(\dfrac{dy}{dt}\)'s calculation, \(y\) when \(x=9\) remains needed. Resorting to our Pythagorean setup discovers \(y\), substituting \(x(t)=9\).

\[(x(t))^2+ (y(t))^2=z^2\]

\[9^2+ (y(t))^2=10^2\]

\[ (y(t))^2=19\]

\[ (y(t))=\sqrt{19}\]

Inserting \(y(t)\)'s value to solve for \(\dfrac{dy}{dt}\).


\[\dfrac{dy}{dt}=-\dfrac{36}{2 \sqrt{19}}\]



The descent signified by a negative conveys the ladder's downward movement direction. Thus, the ladder's descent rate is at \(4.129ft/s\) upon the base being \(9ft\) apart. The horizontal shift pace tangibly corroborates with our discovered rate.

A balloon, assuming a perfect sphere, inflates at \(3cm^2/s\). With its radius at \(4cm\), the expansion rate of the radius is queried.

Step 1: Diagram Visualization

Commencing with a diagram facilitates a grasp on the situation.

Our diagram mandates finding the radius change rate, with the known being the volume change rate.

Step 2: Discriminating Between Knowns and Goals

The problem stipulates the balloon's volume expansion rate as \(3cm^3/s\), seeking the radius's growth rate at a \(4cm\) radius. Assigning variable annotations to our knowns:




Step 3: Employing Sphere Volume Formula

Considering the balloon's shape, the spherical volume formula aids our analysis.

\[V=\dfrac{4}{3} \pi \cdot r^3\]

Step 4: Executing Implicit Differentiation

Differentiating the volume formula time-wise through implicit differentiation aligns change rates.

\[\dfrac{d}{dt}[V]=\dfrac{d}{dt}\left(\dfrac{4}{3} \pi \cdot r^3 \right)\]

\[\dfrac{dV}{dt}=4 \cdot \pi \cdot r^2 \dfrac{dr}{dt}\]

Step 5: Inclusion of Known Variables

The quest is the radius change rate discovery:


With \(r=4cm\) and \(\dfrac{dV}{dt}=3cm^3/s\), substituting these into our formulation:

\[3=4\pi \cdot 4^2 \dfrac{dr}{dt}\]

\[\dfrac{dr}{dt} \approx 0.01492 cm/s \]

\[\dfrac{dr}{dt} \approx 0.015 cm/s \]

A positive growth rate confirms the radius's enlargement. Consequently, the radius enlarges at approximately \(0.015cm/s\). Despite subtleness, this rate aligns with the volume's gradual expansion.

  • Related rates questions generally pertain to discovering one variable's change rate by associating it with other variables whose change rates are established.
  • Efficiently solving related rates problems allows for expressing one rate of change in terms of another, generally more straightforward, rate.
  • Despite the individuality of each related rates challenge, adhering to a uniform strategy that involves:
    • Distinguishing known from unknown quantities
    • Depicting the situation via diagrams
    • Linking variables using equations
    • Resolving through implicit differentiation (timewise)
    • Integrating given data

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