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Question

For the non-linear differential equation $\frac{dy}{dx} = y^2 - y$ with the initial condition $y(0) = 0.5$, the value of $y(3)$ is nearest to _________.

The correct answer is
0.047

Solving the Non-linear Differential Equation

We need to solve the first-order non-linear ordinary differential equation (ODE):

d d x y = y 2 - y $

with the initial condition $y(0) = 0.5$. We aim to find the value of $y(3)$.

Separating Variables

This is a separable ODE. Rearranging the terms gives:

d y y 2 - y = d x $

Factor the denominator:

d y y ( y - 1 ) = d x $

Integration using Partial Fractions

Apply partial fraction decomposition to the left side:

1 y ( y - 1 ) = A y + B y - 1 $

Solving for A and B yields $A = -1$ and $B = 1$. Thus:

( 1 y - 1 - 1 y ) d y = d x $

Integrate both sides:

( 1 y - 1 - 1 y ) d y = d x $

This results in:

ln | y - 1 | - ln | y | = x + C $

Combining the logarithms:

ln | y - 1 y | = x + C $

Applying the Initial Condition

Use $y(0) = 0.5$ to find the constant $C$:

ln | 0.5 - 1 0.5 | = 0 + C $ ln | - 0.5 0.5 | = C $ ln | - 1 | = C ln ( 1 ) = C C = 0 $

The equation becomes:

ln | y - 1 y | = x $

Solving for y(x)

Exponentiating both sides:

| y - 1 y | = e x $

Since $y(0) = 0.5$ is between 0 and 1, and the ODE's right side $y^2-y$ is negative for $0 < y < 1$, the solution $y(x)$ remains between 0 and 1. Therefore, $y-1$ is negative, and $\frac{y-1}{y}$ is negative. So, $|\frac{y-1}{y}| = -(\frac{y-1}{y}) = \frac{1-y}{y}$.

1 - y y = e x $

Rearrange to solve for $y$:

1 - y = y e x $ 1 = y + y e x $ 1 = y ( 1 + e x ) $ y ( x ) = 1 1 + e x $

Calculating y(3)

Substitute $x = 3$ into the solution:

y ( 3 ) = 1 1 + e 3 $

Using the approximate value $e^3 \approx 20.0855$:

y ( 3 ) 1 1 + 20.0855 = 1 21.0855 $

Calculate the final value:

y ( 3 ) 0.04741 $

The value $y(3) \approx 0.04741$ is nearest to 0.047.

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Important Questions from First Order Equations

  1. For the equation \(\frac{{dy}}{{dx}} + 7{x^2}y = 0\) , if y(0) = \(\frac{{3}}{{7}}\) , then the value of y(1) is

  2. The differential equation \(\frac{{dy}}{{dx}} + 4y = 5\) is valid in the domain 0 ≤ x ≤ 1 with y (0) = 2.25 The solution of the differential equation is

  3. The derivative of f(x) = cos(x) can be estimated using the approximation \(f'\left( x \right) = \frac{{f\left( {x + h} \right) - f\left( {x - h} \right)}}{{2h}}\) . The percentage error is calculated as \(\left( {\frac{{Exact\;value - Approximate\;value}}{{Exact\;value}}} \right) \times 100\). The percentage error in the derivative of f(x) at x = π/6 radian, choosing h = 0.1 radian, is

  4. The general solution of the differential equation \(\frac{{dy}}{{dx}} = \cos \left( {x + y} \right)\), with c as a constant, is

  5. Which one of the following is the general solution of the first order differential equation

    \(\frac{{dy}}{{dx}} = {\left( {x + y - 1} \right)^2}\) , where x, y are real?

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