Differential Equations Quiz: Eulers Method

What this quiz covers

This quiz focuses on Eulers Method, giving you a quick way to practice the rules, question types, and explanations that matter most for Differential Equations.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. $E/9$
2. $E/3$ (correct answer)
3. $3E$
4. $E/\sqrt{3}$

Explanation: Euler's method is a first-order method, which means its global truncation error is approximately proportional to the step size $h$. We can write this relationship as $E \approx C \cdot h$ for some constant $C$. Let the initial error be $E_1$ with step size $h_1=h$, so $E_1 = E \approx C \cdot h$. Let the new error be $E_2$ with step size $h_2 = h/3$. Then $E_2 \approx C \cdot h_2 = C \cdot (h/3) = (C \cdot h)/3 \approx E/3$. Therefore, reducing the step size by a factor of 3 reduces the global error by approximately the same factor.

Question 2

1. The step size $h$ must be variable for the errors to be additive, but it is typically kept constant.
2. Local error is of order $O(h^2)$ while global error is of order $O(h)$, and these orders are mathematically incompatible.
3. Each step's calculation begins from a point that is already an approximation, not a point on the true solution curve. (correct answer)
4. Round-off errors from computer arithmetic accumulate and become larger than the local truncation errors.

Explanation: Local error at a step is defined as the error introduced in that single step, under the assumption that the starting point of the step lies on the true solution curve. However, in practice, after the first step, every subsequent step begins from a point $(x_n, y_n)$ that is already an approximation and is not on the true curve. The slope $f(x_n, y_n)$ is calculated at this approximate point, not at the correct point $(x_n, y(x_n))$ on the true solution. This propagation and compounding of errors from previous steps is the fundamental reason why the total global error is not simply the sum of the individual local errors.

Question 3

1. $17/3$ (correct answer)
2. $17/7$
3. $10/3$
4. $5$

Explanation: First, rewrite the differential equation in the form $y' = f(x, y)$: $y' = 3x - 2y$. The initial point is $(x_0, y_0) = (1, y_0)$, the step size is $h=0.2$, and the result of the first step is $y_1 = y(1.2) \approx 4$. The formula for one step of Euler's method is $y_1 = y_0 + h \cdot f(x_0, y_0)$. Substituting the known values: $4 = y_0 + 0.2 \cdot f(1, y_0)$. The function is $f(1, y_0) = 3(1) - 2y_0 = 3 - 2y_0$. So the equation becomes: $4 = y_0 + 0.2(3 - 2y_0)$. Now, we solve for $y_0$: $4 = y_0 + 0.6 - 0.4y_0$ $3.4 = 0.6y_0$ $y_0 = 3.4 / 0.6 = 34/6 = 17/3$.

Question 4

1. $y_n$ approaches 0.
2. $y_n$ approaches a non-zero constant.
3. $y_n$ grows without bound.
4. $y_n$ oscillates with increasing amplitude. (correct answer)

Explanation: The recurrence relation for Euler's method applied to $y'=-2y$ is $y_{n+1} = y_n + h(-2y_n) = y_n(1-2h)$. This is a geometric sequence with ratio $r = 1-2h$. The long-term behavior depends on the value of this ratio. With $h=1.2$, the ratio is $r = 1 - 2(1.2) = 1 - 2.4 = -1.4$. The sequence of approximations is given by $y_n = y_0(r)^n = 1(-1.4)^n$. Since the base of the exponentiation, -1.4, is negative, the sign of $y_n$ will alternate between positive and negative, meaning the sequence oscillates. Since the magnitude of the base, $|-1.4| = 1.4 > 1$, the amplitude of these oscillations will increase without bound. Therefore, $y_n$ oscillates with increasing amplitude.

Question 5

1. $-1/2$ (correct answer)
2. $0$
3. $-1$
4. $-\infty$

Explanation: The problem asks for the limit of the Euler's method approximation as the number of steps $n$ goes to infinity. As $n \to \infty$, the step size $h = (1-0)/n$ approaches 0. In the limit as $h \to 0$, the approximation from Euler's method converges to the true solution. Therefore, we need to find the true value of $y(1)$. The differential equation is separable: $dy/dx = y^2 \implies y^{-2} dy = dx$. Integrating both sides gives $-y^{-1} = x + C$, or $y(x) = -\frac{1}{x+C}$. We use the initial condition $y(0)=-1$ to find $C$: $-1 = -\frac{1}{0+C} \implies C=1$. The true solution is $y(x) = -\frac{1}{x+1}$. The value of the solution at $x=1$ is $y(1) = -\frac{1}{1+1} = -\frac{1}{2}$. Since $\lim_{n \to \infty} y_n = y(1)$, the limit is $-1/2$.

Question 6

1. $0.05$
2. $0.20$
3. $0.15$
4. $0.10$ (correct answer)

Explanation: When you encounter Euler's method problems, you're working with a numerical technique that approximates solutions to differential equations by taking small linear steps. The key is understanding how the step size affects the approximation accuracy. For the IVP $y' = 2x$, $y(0) = 0$, Euler's method uses the formula $y_{n+1} = y_n + h \cdot f(x_n, y_n)$, where $f(x,y) = 2x$. Starting at $(0,0)$, each step adds $h \cdot 2x_n$ to the current $y$-value. With $n = \frac{1}{h}$ steps to reach $x = 1$, the approximation becomes: $y_1 \approx h[2(0) + 2(h) + 2(2h) + \ldots + 2((n-1)h)]$ $= 2h^2[0 + 1 + 2 + \ldots + (n-1)] = 2h^2 \cdot \frac{(n-1)n}{2} = h^2n(n-1)$ Since $n = \frac{1}{h}$, this becomes $h^2 \cdot \frac{1}{h} \cdot (\frac{1}{h} - 1) = h(\frac{1}{h} - 1) = 1 - h$. Setting $1 - h = 0.9$ gives us $h = 0.1$, which is answer choice D. Choice A ($h = 0.05$) would give $y(1) \approx 0.95$, choice B ($h = 0.20$) would give $y(1) \approx 0.8$, and choice C ($h = 0.15$) would give $y(1) \approx 0.85$. Strategy tip: For Euler's method problems, always set up the systematic calculation pattern first. The arithmetic series that emerges from the step-by-step process often simplifies dramatically, making these problems more manageable than they initially appear.

Question 7

1. $-1$
2. $-3$
3. 2 (correct answer)
4. 3

Explanation: We are given $(x_0, y_0) = (0, 1)$, $h=1$, and $f(x, y) = x - 2y$. The formula for Euler's method is $y_{n+1} = y_n + h \cdot f(x_n, y_n)$. Step 1: Find $y_1 \approx y(1)$. $x_0 = 0, y_0 = 1$. The slope at this point is $f(0, 1) = 0 - 2(1) = -2$. $y_1 = y_0 + h \cdot f(x_0, y_0) = 1 + 1 \cdot (-2) = -1$. So, the first approximation point is $(x_1, y_1) = (1, -1)$. Step 2: Find $y_2 \approx y(2)$. $x_1 = 1, y_1 = -1$. The slope at this new point is $f(1, -1) = 1 - 2(-1) = 1 + 2 = 3$. $y_2 = y_1 + h \cdot f(x_1, y_1) = -1 + 1 \cdot (3) = 2$. Thus, the second approximation is $y_2 = 2$.

Question 8

1. $|E_y(1)| > |E_z(1)|$
2. $|E_y(1)| < |E_z(1)|$ (correct answer)
3. $|E_y(1)| \approx |E_z(1)|$
4. The relationship cannot be determined without knowing the value of $h$.

Explanation: The error in Euler's method is closely related to the magnitude of the second derivative of the solution, which measures the curve's concavity. A larger second derivative implies the curve bends away from its tangent line more sharply, leading to a larger error in the linear approximation. For IVP 1, the solution is $y(x) = e^x$. The second derivative is $y''(x) = e^x$. For IVP 2, the solution is $z(x) = e^{2x}$. The second derivative is $z''(x) = 4e^{2x}$. Over the interval $[0, 1]$, the value of $z''(x) = 4e^{2x}$ is always significantly larger than $y''(x) = e^x$. Because the solution curve for IVP 2 is much more concave up than the solution for IVP 1, its approximation by straight line segments will be less accurate. Therefore, the magnitude of the global error for IVP 2 will be larger than that for IVP 1, meaning $|E_z(1)| > |E_y(1)|$.

Question 9

1. $y_E(x)$ will be an underestimate of the true solution $y(x)$. (correct answer)
2. $y_E(x)$ will be an overestimate of the true solution $y(x)$.
3. $y_E(x)$ will be equal to the true solution $y(x)$.
4. The relationship between $y_E(x)$ and $y(x)$ cannot be determined without knowing the value of $h$.

Explanation: The error behavior of Euler's method depends on the concavity of the solution curve, which is determined by the sign of the second derivative, $y''$. Given $y' = x - y^2$, we can find $y''$ using implicit differentiation with respect to $x$: $y'' = \frac{d}{dx}(x - y^2) = 1 - 2y \cdot y'$. Substituting the expression for $y'$, we get $y'' = 1 - 2y(x - y^2) = 1 - 2xy + 2y^3$. At the initial point $(x_0, y_0) = (0, 1)$, we have $y''(0) = 1 - 2(0)(1) + 2(1)^3 = 3$. Since $y'' > 0$ at the initial point, the solution curve is concave up. Euler's method uses tangent line segments to approximate the curve. For a concave up function, the tangent lines lie below the curve, resulting in an underestimate.

Question 10

1. The method will initially produce an underestimate because $y(0)=0$.
2. The method will initially produce an overestimate because the solution is initially concave down. (correct answer)
3. The method will produce the exact solution because $\cos(x)$ is an even function.
4. The error will initially be zero and then become an underestimate for all $x>0$.

Explanation: The nature of the error in Euler's method (overestimate vs. underestimate) depends on the concavity of the solution curve, determined by the sign of $y''$. We have $y' = \cos(x) - y$. Differentiating with respect to $x$ gives $y'' = -\sin(x) - y'$. Substituting $y'$ yields $y'' = -\sin(x) - (\cos(x) - y) = y - \sin(x) - \cos(x)$. At the initial point $(x_0, y_0) = (0, 0)$, the second derivative is $y''(0) = 0 - \sin(0) - \cos(0) = 0 - 0 - 1 = -1$. Since $y'' < 0$ at the starting point, the solution curve is concave down. Euler's method follows tangent lines, which lie above a concave down curve. Therefore, for $x$ slightly greater than 0, the approximation will be an overestimate.

Question 11

1. $0.005$
2. $0.01$
3. $0.02$ (correct answer)
4. $0.04$

Explanation: Euler's method is a first-order method, which means its global truncation error is approximately proportional to the step size $h$. Let the error be $E_h \approx C \cdot h$ for some constant $C$. We are given that for $h_1 = 0.2$, the error $E_{0.2}$ is $0.08$. We can estimate $C$ as $C \approx E_{0.2} / h_1 = 0.08 / 0.2 = 0.4$. Now, we can predict the error for the new step size $h_2 = 0.05$: $E_{0.05} \approx C \cdot h_2 = 0.4 \cdot 0.05 = 0.02$. Alternatively, since the error is proportional to $h$, the ratio of the new error to the old error should be the same as the ratio of the new step size to the old step size: $E_{new} / E_{old} = h_{new} / h_{old}$. So, $E_{new} = E_{old} \cdot (h_{new} / h_{old}) = 0.08 \cdot (0.05 / 0.2) = 0.08 \cdot (1/4) = 0.02$.

Question 12

1. $1.20$
2. $1.32$ (correct answer)
3. $1.60$
4. $1.28$

Explanation: We need to perform two steps of Euler's method. Let $(x_0, y_0) = (0, 2)$ and $h=0.2$. The formula is $y_{n+1} = y_n + h \cdot f(x_n, y_n)$. Step 1: Find $y_1 \approx y(0.2)$. $x_0 = 0$, $y_0 = 2$. The slope is $f(x_0, y_0) = 0 - 2 = -2$. $y_1 = y_0 + h \cdot f(x_0, y_0) = 2 + 0.2(-2) = 2 - 0.4 = 1.6$. So, $(x_1, y_1) = (0.2, 1.6)$. Step 2: Find $y_2 \approx y(0.4)$. $x_1 = 0.2$, $y_1 = 1.6$. The slope is $f(x_1, y_1) = 0.2 - 1.6 = -1.4$. $y_2 = y_1 + h \cdot f(x_1, y_1) = 1.6 + 0.2(-1.4) = 1.6 - 0.28 = 1.32$. Thus, the approximation for $y(0.4)$ is $1.32$.

Question 13

1. $0.10$
2. $0.15$
3. $0.17$
4. $0.20$ (correct answer)

Explanation: Euler's method formula for one step is $y_1 = y_0 + h \cdot f(x_0, y_0)$. We are given the initial condition $(x_0, y_0) = (0, 1)$ and the result $y_1 = 1.2$. The function defining the slope is $f(x, y) = 2x + y$. First, we evaluate the slope at the initial point: $f(x_0, y_0) = f(0, 1) = 2(0) + 1 = 1$. Now, we plug the known values into the formula: $1.2 = 1 + h \cdot (1)$. Solving for $h$, we get $1.2 - 1 = h$, which means $h = 0.2$.

Question 14

1. $-0.04$
2. $0.00$
3. $0.04$ (correct answer)
4. $0.24$

Explanation: First, we calculate $A_1$ with $h=0.2$. The initial point is $(x_0, y_0) = (0, 1)$. The slope is $f(0, 1) = 2(1) = 2$. $A_1 = y_1 = y_0 + h \cdot f(x_0, y_0) = 1 + 0.2(2) = 1.4$. Next, we calculate $A_2$ with $h=0.1$. This requires two steps. Step 1: from $x=0$ to $x=0.1$. $(x_0, y_0) = (0, 1)$. The slope is $f(0, 1) = 2(1) = 2$. $y_1 = 1 + 0.1(2) = 1.2$. The new point is $(0.1, 1.2)$. Step 2: from $x=0.1$ to $x=0.2$. $(x_1, y_1) = (0.1, 1.2)$. The slope is $f(0.1, 1.2) = 2(1.2) = 2.4$. $A_2 = y_2 = y_1 + h \cdot f(x_1, y_1) = 1.2 + 0.1(2.4) = 1.2 + 0.24 = 1.44$. Finally, we find the difference: $A_2 - A_1 = 1.44 - 1.4 = 0.04$.

Question 15

1. The approximation is a guaranteed overestimate.
2. The approximation is a guaranteed underestimate.
3. The approximation is exact because the derivative function is simple.
4. The approximation is neither a guaranteed overestimate nor a guaranteed underestimate. (correct answer)

Explanation: When analyzing Euler's method error behavior, you need to understand how the method's linear approximation interacts with the actual curve's concavity over the entire interval. Euler's method uses tangent line segments to approximate the solution. Whether this creates an overestimate or underestimate depends on the concavity of the true solution. For $y' = \cos(x)$ with $y(0) = 0$, the exact solution is $y = \sin(x)$. The key insight is examining $y'' = -\sin(x)$ over $[0,4]$. The second derivative changes sign multiple times: it's negative on $(0,\pi)$, positive on $(\pi,2\pi)$, negative on $(2\pi,3\pi)$, and positive on $(3\pi,4]$ since $4 > 3\pi$. When the solution is concave down ($y'' < 0$), Euler's method overestimates; when concave up ($y'' > 0$), it underestimates. Answer A is wrong because the solution isn't always concave down over $[0,4]$ - there are intervals where $y'' > 0$. Answer B is wrong for the same reason in reverse - the solution isn't always concave up. Answer C is incorrect because Euler's method is only exact for linear functions, and $\sin(x)$ is nonlinear despite having a "simple" derivative. Answer D is correct because the concavity changes multiple times over the interval, so Euler's method sometimes overestimates and sometimes underestimates, making the cumulative error at $x = 4$ unpredictable in sign. Study tip: For Euler's method error analysis, always check the second derivative of the exact solution to determine concavity changes over the entire interval.

Question 16

1. $3.15$ (correct answer)
2. $3.00$
3. $2.85$
4. $3.30$

Explanation: When you encounter Euler's method problems, you're applying a fundamental numerical technique that approximates solutions to differential equations by taking small linear steps based on the slope at each point. First, you need to express the differential equation in the form $y' = f(x,y)$. Starting with $2y' - y = 4x$, solve for $y'$: $y' = \frac{y + 4x}{2}$. This gives you the slope function. Euler's method uses the formula: $y_{n+1} = y_n + h \cdot f(x_n, y_n)$, where $h$ is the step size. With your initial condition $y(0) = 3$, you have $x_0 = 0$, $y_0 = 3$, and $h = 0.1$. Calculate the slope at the initial point: $f(0,3) = \frac{3 + 4(0)}{2} = \frac{3}{2} = 1.5$ Apply Euler's formula: $y_1 = y_0 + h \cdot f(x_0, y_0) = 3 + 0.1(1.5) = 3 + 0.15 = 3.15$ This confirms answer A) $3.15$ is correct. Answer B) $3.00$ would result from forgetting to add the increment ($h \cdot f$) to the initial value. Answer C) $2.85$ likely comes from subtracting instead of adding the increment. Answer D) $3.30$ suggests doubling the correct increment, possibly from miscalculating the slope function. Remember: Euler's method always moves in the direction of the slope. Double-check your slope calculation by carefully solving for $y'$, and verify you're adding (not subtracting) the increment $h \cdot f(x,y)$.

Question 17

1. The error is exactly zero for any step size $h$. (correct answer)
2. The error is non-zero and is proportional to the step size $h$.
3. The error is non-zero and is proportional to the square of the step size, $h^2$.
4. The error is positive because the solution is an increasing function.

Explanation: The exact solution to the differential equation $y' = 3$ with $y(1) = 5$ can be found by direct integration: $y(x) = 3x + C$. Using the initial condition, $5 = 3(1) + C$, which gives $C=2$. The exact solution is the line $y(x) = 3x + 2$. Euler's method approximates the solution curve by taking steps along tangent lines. For a linear function, the tangent line at any point is the function itself. Therefore, each step of Euler's method will land exactly on the true solution line, regardless of the step size $h$. As a result, the error will be exactly zero.

Question 18

1. The approximation will be an overestimate of the true solution.
2. The approximation will be an underestimate of the true solution. (correct answer)
3. The approximation will be exact because the differential equation is linear.
4. It is not possible to determine if the approximation is an overestimate or underestimate without knowing the step size $h$.

Explanation: To determine if Euler's method overestimates or underestimates, we examine the concavity of the solution curve by finding the second derivative, $y''$. Given $y' = y - x^2$, we differentiate with respect to $x$: $y'' = \frac{d}{dx}(y - x^2) = y' - 2x$. Substituting the expression for $y'$, we get $y'' = (y - x^2) - 2x = y - x^2 - 2x$. We evaluate the sign of $y''$ at the initial point $(0, 2)$: $y''(0) = 2 - 0^2 - 2(0) = 2$. Since $y''(0) > 0$, the solution curve is concave up near $x=0$. Euler's method works by following tangent lines. For a concave up curve, the tangent line at any point lies below the curve. Therefore, each step of Euler's method will fall below the true solution, resulting in an underestimate.