Question 1

Do I need a constant of integration $+\,c$ when finding the integrating factor?

Accepted Answer

No. While technically $\int P(x) dx$ has a constant, $e^{P(x)dx + c} = e^{\int P(x)dx} \cdot e^c$. This $e^c$ is just another constant multiplier that would apply to the whole equation and eventually get absorbed into the final constant of integration anyway. So, for simplicity, we omit it when calculating the I.F.

Question 2

What's the difference between a general and a particular solution?

Accepted Answer

A general solution has the constant $+\,c$ and represents an entire family of functions that satisfy the differential equation. A particular solution is a single, unique function from that family, found by using a specific point (a boundary condition) to determine the exact value of $c$.

Question 3

How do I deal with $\ln|f(y)|$ when solving for $y$?

Accepted Answer

After integrating to get $\ln|f(y)| = g(x) + c$, you exponentiate both sides: $|f(y)| = e^{g(x)+c} = e^{g(x)}e^c$. We replace the constant $e^c$ with a new constant, say $A$, so $|f(y)| = Ae^{g(x)}$. This means $f(y) = \pm Ae^{g(x)}$. You can absorb the $\pm$ into the constant $A$. The sign of $A$ is determined by the initial conditions.

Question 4

My answer for a modelling question is $y = 5e^{kt}$ but the mark scheme has $y = Ae^{kt}$. Why?

Accepted Answer

This often happens when the initial value is used at different stages. If you solve for the constant of integration $c$ early, you might get $\ln(y) = kt + \ln(5)$, which gives $y = e^{kt + \ln(5)} = e^{kt}e^{\ln(5)} = 5e^{kt}$. If you write the general solution as $y = Ae^{kt}$ and then substitute $t=0, y=5$, you get $5 = Ae^0$, so $A=5$. Both methods are correct and lead to the same result.

Question 5

Can I always use separation of variables?

Accepted Answer

No, it only works if the equation can be written in the form $dy/dx = f(x)g(y)$. For linear equations like $dy/dx + 2y = x$, you cannot separate the $2y$ term from the $x$ term, so you must use the integrating factor method.

Differential equations

What you need to know

Equations of Change