Reasoning Using Slope Fields

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AP Calculus BC › Reasoning Using Slope Fields

Questions 1 - 10
1

For the slope field of $\frac{dy}{dx}=x^2-y^2$, at which point is the slope zero?

$(0,1)$

$(0,0)$

$(1,1)$

$(2,1)$

$(1,0)$

Explanation

This problem tests finding critical points in slope fields. Setting dy/dx = 0 gives us x² - y² = 0, or x² = y². Among the given points, only (1,1) satisfies this equation since 1² = 1². The point (1,0) seems plausible but gives slope = 1² - 0² = 1 ≠ 0. To locate horizontal tangents in slope fields, solve the equation obtained by setting the differential equation equal to zero.

2

For the slope field of $\frac{dy}{dx}=\frac{y}{x}$, which statement about solutions in quadrant I is true?

Every solution crosses the $x$-axis.

Every solution is decreasing for $x>0$.

Every solution has horizontal tangents for $y>0$.

Every solution is a line through the origin.

Every solution has constant slope $1$.

Explanation

This question analyzes solution behavior for separable equations in slope fields. The differential equation dy/dx = y/x can be rewritten as dy/y = dx/x, which integrates to ln|y| = ln|x| + C. This simplifies to y = kx for some constant k, meaning all solutions are lines through the origin. The decreasing option (D) fails because these linear solutions have positive slope k in quadrant I. For equations of the form dy/dx = g(y)/h(x), separation of variables often reveals the solution structure.

3

For the slope field of $\frac{dy}{dx}=x(1-y)$, at which point is the slope negative?

$(2,2)$

$(-2,0)$

$(0,5)$

$(2,0)$

$(-2,2)$

Explanation

This problem tests sign analysis in two-variable slope fields. At point (2,2), we calculate dy/dx = 2(1-2) = 2(-1) = -2 < 0, confirming negative slope. At (-2,2), we get dy/dx = -2(1-2) = -2(-1) = 2 > 0, showing positive slope instead. The pattern dy/dx = x(1-y) gives negative slopes when x and (1-y) have opposite signs. To determine slope signs efficiently, factor the differential equation and analyze the sign of each factor at the given point.

4

For the slope field of $\frac{dy}{dx}=1-y$, what is the long-term behavior of solutions with $y(0)=3$?

They remain constant at $y=3$.

They increase without bound as $x\to\infty$.

They approach $y=1$ as $x\to\infty$.

They oscillate between $y=0$ and $y=2$.

They approach $y=0$ as $x\to\infty$.

Explanation

This problem examines long-term behavior using slope field analysis. The differential equation dy/dx = 1 - y has equilibrium at y = 1 (where dy/dx = 0). For y(0) = 3 > 1, we have dy/dx = 1 - 3 = -2 < 0, so the solution decreases toward the equilibrium y = 1. The unbounded growth option (A) fails because dy/dx < 0 whenever y > 1, preventing increase. For autonomous equations dy/dx = f(y), stable equilibria attract nearby solutions.

5

For the slope field of $\frac{dy}{dx}=\sin x$, which statement about solution curves is true?

Solutions are undefined where $x=0$.

All solutions are vertical translations of one another.

All solutions pass through $(0,0)$.

Solutions have the same slope along each horizontal line.

All solutions are horizontal translations of one another.

Explanation

This question examines how slope field patterns determine solution relationships. Since dy/dx = sin x depends only on x (not on y), all points with the same x-coordinate have identical slopes. This means solution curves maintain the same vertical spacing everywhere, making them vertical translations of each other. The horizontal translation option (B) fails because sin x has different values at different x-coordinates. For differential equations of the form dy/dx = f(x), all solutions differ by only a vertical shift.

6

For the slope field of $\frac{dy}{dx}=\frac{1}{1+y^2}$, which statement about all solution curves is true?

They have positive slope everywhere.

They are decreasing everywhere.

They are undefined when $y=0$.

They have slope $0$ when $y=0$.

They have slope between $-1$ and $0$ everywhere.

Explanation

This question examines global properties of slope fields. Since 1 + y² ≥ 1 for all real y, we have 0 < dy/dx ≤ 1 everywhere, meaning all solutions have positive slope and are increasing functions. The slope equals 1 when y = 0 and approaches 0 as |y| → ∞. The negative slope option (B) fails because 1/(1+y²) cannot be negative. For rational functions in slope fields, analyze the sign and bounds of the expression to determine universal solution properties.

7

For the slope field of $\frac{dy}{dx}=x-y$, which statement about the solution through $(0,1)$ is true?

It is increasing at $(0,1)$.

It is undefined at $(0,1)$.

It has a vertical tangent at $(0,1)$.

It is decreasing at $(0,1)$.

It has a horizontal tangent at $(0,1)$.

Explanation

This question tests reasoning about solution behavior using slope fields. At the point (0,1), we calculate the slope: dy/dx = 0 - 1 = -1. Since the slope is negative, the solution curve must be decreasing as it passes through this point. The horizontal tangent option (A) would require slope = 0, which contradicts our calculation. To analyze slope fields systematically, always substitute the given point coordinates into the differential equation to determine the exact slope value.

8

For $\frac{dy}{dx}=x+y$, which region of the slope field has negative slopes?

Points with $y=0$

Points with $y>-x$

Points with $y=-x$

Points with $y<-x$

Points with $x=0$

Explanation

Reasoning using slope fields involves determining regions where slopes are positive, negative, or zero to understand solution monotonicity. For dy/dx = x + y, slopes are negative precisely when x + y < 0, or y < -x. In this region below the line y = -x, solution curves decrease as they follow negative tangents. Above the line, positive slopes indicate increasing behavior. A tempting distractor is points with y > -x, but this fails as slopes are positive there, not negative. A transferable strategy for slope fields is to plot the zero-slope isocline to divide the plane into regions of consistent slope sign.

9

In the slope field for $\frac{dy}{dx}=\frac{y}{1+x^2}$, what is the sign of the slope at $(1,-2)$?

Negative

Zero

Undefined

Cannot be determined from the differential equation

Positive

Explanation

This question tests reasoning using slope fields by determining the sign of the slope at a specific point. In the slope field for dy/dx = y/(1 + x²), the denominator 1 + x² is always positive, so the sign of the slope matches the sign of y. At (1,-2), y = -2 < 0, so the slope is negative, indicating the solution curve is decreasing there. This field's slopes are positive above the x-axis and negative below, scaled by the positive denominator. A tempting distractor is positive, which would be true if y were positive, but here y is negative. A transferable slope-field strategy is to factor the differential equation to determine the sign based on the signs of numerator and denominator at the point.

10

In the slope field for $\frac{dy}{dx}=y(2-y)$, which statement about solutions with $0<y<2$ is correct?

They decrease for all $x$

They alternate increasing and decreasing periodically

They are undefined at $y=1$

They increase for all $x$

They have slope $0$ for all $x$

Explanation

This question tests reasoning using slope fields by describing the behavior of solutions in a specific region. In the slope field for $\frac{dy}{dx} = y(2 - y)$, for $0 < y < 2$, both $y > 0$ and $(2 - y) > 0$, so the product is positive, meaning all slopes are positive and solutions are increasing for all $x$. As $y$ approaches 2 from below, the slope approaches 0, so solutions increase toward the equilibrium at y = 2 asymptotically. Below $y = 0$ or above $y = 2$, slopes are negative, leading to different behaviors like decreasing toward equilibria. A tempting distractor is that they decrease for all $x$, which might occur if one flips the sign of $(2 - y)$, but it's positive in this interval. A transferable slope-field strategy is to analyze the sign of $\frac{dy}{dx}$ in different regions divided by equilibria to predict whether solutions increase or decrease.

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