kids' differential equation

\(\displaystyle (y^2 + 1) \ dx = y \sec^2 x \ dy\)
\(\displaystyle (y^2 + 1) \ dx = y \sec^2 x \ dy\)


\(\displaystyle \frac{1}{\sec^2 x} \ dx = \frac{y}{y^2 + 1} \ dy\)


\(\displaystyle \cos^2 x \ dx = \frac{y}{y^2 + 1} \ dy\)


\(\displaystyle \int \cos^2 x \ dx = \int \frac{y}{y^2 + 1} \ dy\)


\(\displaystyle \frac{1}{2}\int (1 + \cos 2x) \ dx = \frac{1}{2}\int \frac{2y}{y^2 + 1} \ dy\)


\(\displaystyle \textcolor{blue}{\frac{x}{2} + \frac{\sin 2x}{4} = \frac{1}{2}\ln\left(y^2 + 1\right) + c}\)
 
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