Problem 35 - Calculus

Question

Are the statements true or false? Give an explanation for your answer. The solutions of the differential equation \(d y / d x=x^{2}+\) \(y^{2}+1\) are concave up at every point.

Step-by-Step Solution

Answer

False. The second derivative can be negative for some values of \( x \) and \( y \).

Step 1: Understanding Concavity

Solutions of a differential equation being concave up means their second derivatives are positive. We need to work with the given differential equation to understand this behavior.

Step 2: Given Differential Equation

The differential equation given is \( \frac{dy}{dx} = x^2 + y^2 + 1 \).

Step 3: Second Derivative Implication

To analyze concavity, find the second derivative \( \frac{d^2 y}{d x^2} \). The function is concave up where \( \frac{d^2 y}{d x^2} > 0 \).

Step 4: Find the Second Derivative

To find \( \frac{d^2 y}{d x^2} \), differentiate \( \frac{dy}{dx} = x^2 + y^2 + 1 \) with respect to \( x \). This yields \( \frac{d^2 y}{d x^2} = 2x + 2y \frac{dy}{dx} \).

Step 5: Substitute \( \frac{dy}{dx} \)

Substitute \( \frac{dy}{dx} = x^2 + y^2 + 1 \) into the second derivative: \( \frac{d^2 y}{d x^2} = 2x + 2y(x^2 + y^2 + 1) \).

Step 6: Condition for Concavity

Analyze when \( 2x + 2y(x^2 + y^2 + 1) > 0 \). This is not always true for all \( x \) and \( y \).

Step 7: Conclusion on Concavity

Since \( \frac{d^2 y}{d x^2} = 2x + 2y(x^2 + y^2 + 1) \) can be negative for some values of \( x \) and \( y \), the statement is false.

Key Concepts

Concavity and its Importance

Concavity is an important concept in calculus that helps us understand the shape of a function's graph. It is related to the curvature or bending of the graph. A function is said to be *concave up* at a point if, when you draw a tangent line at that point, the graph of the function lies above the tangent line. This often resembles a bowl that opens upward. Similarly, a function is *concave down* if the graph lies below the tangent line, resembling a bowl that opens downward.

To determine concavity, we rely on the second derivative of the function. If the second derivative is positive (\( \frac{d^2 y}{d x^2} > 0 \)), the function is concave up at that point. If it is negative (\( \frac{d^2 y}{d x^2} < 0 \)), the function is concave down. This is why the second derivative is crucial for analyzing concavity, as it gives us a precise mathematical tool to determine the nature of the function's curvature.

Understanding the Second Derivative

The second derivative of a function is derived by differentiating the first derivative, essentially capturing changes in the rate of change or the *acceleration* of the function. For any function, let's call it \( y=f(x) \), the first derivative \( \frac{dy}{dx} \) tells us the rate of change or slope of the function at any point. Differentiating this result again, we obtain the second derivative \( \frac{d^2y}{dx^2} \).

• If \( \frac{d^2y}{dx^2} > 0 \), the function is accelerating and the slope is increasing, suggesting the graph is curving upwards.
• If \( \frac{d^2y}{dx^2} < 0 \), the function is decelerating, and the slope is decreasing, suggesting the graph is curving downwards.

This is crucial when examining the solutions to differential equations, as it helps identify the concavity and the overall behavior of such solutions without plotting every single point on a graph.

Solving Differential Equations for Shape Insight

When dealing with differential equations, solving them gives us insights into the function \( y \) itself. The example in our exercise involves the equation \( \frac{dy}{dx} = x^2 + y^2 + 1 \). To understand the shape of the solution more deeply, we examine not just the first derivative but also the second derivative.

In the process of analyzing this differential equation, we derived the second derivative as \( \frac{d^2 y}{d x^2} = 2x + 2y(x^2 + y^2 + 1) \). We then explored the conditions under which this expression is greater than zero, as this would imply the solutions are concave up. However, as shown, this inequality does not hold for all values of \( x \) and \( y \), leading us to the conclusion that the function isn't always concave up.

This approach underscores the importance of differential equations as tools for revealing qualitative properties of functions. By understanding both derivatives, students gain a comprehensive view of how solutions behave, leading to better intuition about possible graphs and curves described by these equations.