Question

Consider the implicit differential equation 20%2898%20xy%5E%7B2%7D%20%2B50%20x%5E%7B2%7D%20%29dy%3D0" id="TexFormula1" title="(49 y^{3} + 45 xy) dx + (98 xy^{2} +50 x^{2} )dy=0" alt="(49 y^{3} + 45 xy) dx + (98 xy^{2} +50 x^{2} )dy=0" align="absmiddle" class="latex-formula">. For the integrating factor $x^{p} y^{q}$ of this equation, find p and q. Now multiply the equation by the integrating factor $x^{p} y^{q}$ that you have found and then integrate the resulting equation to get a solution in implicit form.

Answer 1

We're looking for an integrating factor $\mu(x,y)=x^py^q$ such that

$\mu\underbrace{(49y^3+45xy)}_M\,\mathrm dx+\mu\underbrace{(98xy^2+50x^2)}_N\,\mathrm dy=0$

is exact, which would require that

$(\mu M)_y=(\mu N)_x$
$(49x^py^{q+3}+45x^{p+1}y^{q+1})_y=(98x^{p+1}y^{q+2}+50x^{p+2}y^q)_x$
$49(q+3)x^py^{q+2}+45(q+1)x^{p+1}y^q=98(p+1)x^py^{q+2}+50(p+2)x^{p+1}y^q$
$\implies\begin{cases}49(q+3)=98(p+1)\\45(q+1)=50(p+2)\end{cases}\implies p=\dfrac52,q=4$

You can verify that $(\mu M)_y=(\mu N)_x$ if you'd like. With the ODE now exact, we have a solution $F(x,y)=C$ such that

$F_x=\mu M$
$F=\displaystyle\int(49y^3+45xy)x^{5/2}y^4\,\mathrm dx$
$F=10x^{9/2}y^5+14x^{7/2}y^7+f(y)$

$F_y=\mu N$
$50x^{9/2}y^4+98x^{7/2}y^6+f'(y)=98x^{7/2}y^2+50x^{9/2}y^4$
$f'(y)=0$
$\implies f(y)=C$

and so the general solution is

$F(x,y)=10x^{9/2}y^5+14x^{7/2}y^7=C$