If x²y³ = 4/27 and dy/dt = ¹/₂, then what is dx/dt when x = 2?
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4. Applications of Derivatives
Related Rates
Problem 3.8.18a
Textbook Question
Diagonals If x, y, and z are lengths of the edges of a rectangular box, then the common length of the box’s diagonals is s = √(x² + y² + z²).
a. Assuming that x, y, and z are differentiable functions of t, how is ds/dt related to dx/dt, dy/dt, and dz/dt?

1
Start by recognizing that the given formula for the diagonal length s is s = √(x² + y² + z²). This is a function of x, y, and z, which are themselves functions of t.
To find ds/dt, apply the chain rule for differentiation. The chain rule states that if a variable depends on multiple other variables, each of which depends on another variable, you differentiate with respect to each intermediate variable and multiply by the derivative of the intermediate variable with respect to the final variable.
Differentiate s with respect to t: ds/dt = (1/2)(x² + y² + z²)^(-1/2) * (2x dx/dt + 2y dy/dt + 2z dz/dt). This uses the chain rule and the power rule for differentiation.
Simplify the expression: ds/dt = (x dx/dt + y dy/dt + z dz/dt) / √(x² + y² + z²). This is the relationship between ds/dt and the rates of change of x, y, and z with respect to t.
This result shows how the rate of change of the diagonal length s is a weighted sum of the rates of change of the box's edge lengths, where each weight is the corresponding edge length divided by the diagonal length.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Differentiation
Differentiation is a fundamental concept in calculus that deals with finding the rate at which a function changes at any given point. It involves computing the derivative, which represents the slope of the tangent line to the function's graph. In this context, it helps determine how the diagonal length changes with respect to time.
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Chain Rule
The chain rule is a formula for computing the derivative of a composite function. It states that the derivative of a function with respect to a variable can be found by multiplying the derivative of the function with respect to an intermediate variable by the derivative of the intermediate variable with respect to the original variable. This rule is essential for relating ds/dt to dx/dt, dy/dt, and dz/dt.
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Intro to the Chain Rule
Partial Derivatives
Partial derivatives involve taking the derivative of a multivariable function with respect to one variable while keeping the other variables constant. In the context of the diagonal length formula, partial derivatives help in understanding how changes in each edge length (x, y, z) individually affect the diagonal length, which is crucial for applying the chain rule.
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Derivatives
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