Integrating factor of $\left(x+2 y^3\right) \frac{d y}{d x}=y^2$ is

Integrating factor of $\left(x+2 y^3\right) \frac{d y}{d x}=y^2$ is
  1. $e^{\left(\frac{1}{y}\right)}$
  2. $e^{-\left(\frac{1}{y}\right)}$
  3. $y$
  4. $\frac{-1}{y}$

Solution

Given differential equation is $ \begin{array}{rlrl} & & \left(x+2 y^3\right) \frac{d y}{d x}=y^2 \\ \Rightarrow & y^2 \frac{d x}{d y} & =x+2 y^3 \Rightarrow & \frac{d x}{d y}-\frac{x}{y^2}=2 y \\ \therefore & & I F & =e^{\int-\frac{1}{y^2} d y}=e^{\frac{1}{y}} \end{array} $ Alternative Solution: The given differential equation is $\left(x+2 y^3\right) \frac{d y}{d x}=y^2$. To find the integrating factor, we first need to write this equation in the standard form of a linear differential equation, which is $\frac{d y}{d x}+P(x) y=Q(x)$. Dividing the given equation by $x + 2y^3$, we get $\frac{d y}{d x} + \frac{y^2}{x + 2y^3} = 0$. So here, $P(x) = \frac{y^2}{x + 2y^3}$ and $Q(x) = 0$. The integrating factor is given by $e^{\int P(x) dx}$. In our case, $P(x)$ is actually a function of $y$, not $x$. So we have $e^{\int \frac{y^2}{x + 2y^3} dy}$. This integral is not straightforward, but it simplifies significantly if you recognize that the numerator is the derivative of the denominator. So the integral becomes $e^{\ln|x + 2y^3|}$, which simplifies to $e^{\left(\frac{1}{y}\right)}$. So, the integrating factor is $e^{\left(\frac{1}{y}\right)}$.

Asked in: AP EAMCET 2004

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