Question

Решить два уравнения: а) 4xy*dy=(x^2+1)*dyб) x^2*dy=(xy-y^2)*dx

Answer 1

$a_1)\; \; 4xy\cdot dy=(x^2+1)\cdot dx\\\\\int 4y\cdot dy=\int \frac{(x^2+1)\cdot dx}{x}\; \; ,\; \; 4\int y\cdot dy=\int (x+\frac{1}{x})\cdot dx\; ,\\\\4\cdot \frac{y^2}{2}=\frac{x^2}{2}+ln|x|+C\\\\2y^2=\frac{x^2}{2}+ln|x|+C\\\\\\a_2)\; \; 4xy\cdot dx=(x^2+1)\cdot dy\\\\\int \frac{4x\cdot dx}{x^2+1}=\int \frac{dy}{y}\; \; ,\; \; 2\cdot \int \frac{2x\cdot dx}{x^2+1}=\int \frac{dy}{y}\; ,\; \; \Big [\; (x^2+1)'=2x\; \Big ]\\\\2\cdot ln|x^2+1|=ln|y|+ln|C|\\\\(x^2+1)^2=Cy\; \; \to \; \; y=\frac{1}{C}\cdot (x^2+1)^2\; ,\\\\y=C_1\cdot (x^2+1)^2\; ,\; C_1=\frac{1}{C}$

$b)\; \; x^2\cdot dy=(xy-y^2)\cdot dx\\\\\frac{dy}{dx}=\frac{xy-y^2}{x^2}\; \; ,\; \; y'=\frac{y}{x}-(\frac{y}{x})^2\; \; ,\\\\u=\frac{y}{x}\; ,\; \; y=ux\; ,\; \; y'=u'x+u\\\\u'x+u=u-u^2\; \; ,\; \; u'x=-u^2\\\\\frac{du}{dx}\cdot x=-u^2\; \; ,\; \; \int \frac{du}{u^2}=-\int \frac{dx}{x}\; \; ,\\\\\frac{u^{-1}}{-1}=-ln|x|-C\\\\\frac{1}{u}=ln|x|+C\\\\\frac{x}{y}=ln|x|+C\\\\y=\frac{x}{ln|x|+C}$