Problem 112
Medium Linear Equations and Applications

Using appropriately chosen substitutions, integrate…

Using appropriately chosen substitutions, integrate $y'+ay^2=\dfrac{b}{x^2}$.

Explanation of Method

Try a particular solution $y_1=c/x$; matching coefficients gives $c$ as a root of $ac^2-c-b=0$. Reduce via $y=y_1+1/u$; the resulting linear equation for $u$ has two cases depending on whether $2ac=1$.

Step 1: Find $c$

Substituting $y_1=c/x$: $ac^2-c=b$, so $c=\dfrac{1\pm\sqrt{1+4ab}}{2a}$.

Step 2: Reduce via $y=c/x+1/u$

With $Q=0$, $R=-a$: $Q+2Ry_1=-\dfrac{2ac}{x}$, giving $u'-\dfrac{2ac}{x}u=a$.

Step 3a: Case $2ac\ne1$

Integrating factor $x^{-2ac}$: $\dfrac{d}{dx}\!\left(\dfrac{u}{x^{2ac}}\right)=\dfrac{a}{x^{2ac}}$, giving $u=\dfrac{ax}{1-2ac}+Cx^{2ac}$.

Step 3b: Case $2ac=1$

Then $u'-\dfrac{u}{x}=a$, so $\dfrac{d}{dx}\!\left(\dfrac{u}{x}\right)=\dfrac{a}{x}$, giving $u=x(a\ln|x|+C)$.


Result

Final answer:

With $c=\dfrac{1\pm\sqrt{1+4ab}}{2a}$:

Case $2ac\ne1$: \[\boxed{\,y=\frac{c}{x}+\frac{1-2ac}{ax+C(1-2ac)x^{2ac}}}\]

Case $2ac=1$: \[\boxed{\,y=\frac{c}{x}+\frac{1}{x(a\ln|x|+C)}}\]

(both verified symbolically).