Past exam of the mathematics course of the year 1 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 13 / b / Solution

Where $\sin x\ne 0$, divide by it to obtain $y^{\prime }+2\cot xy=\csc x$. The integrating factor is ${\sin }^{2}x$, and the product rule givesThe initial condition gives $C=1$. HenceThe simplified expression has a differentiable extension through $x=0$, with $y(0)=1/2$. Direct substitution verifies the original undivided differential equation there as well: its left side is $2y(0)=1$. Thus the maximal interval containing $\pi /2$ on which this solution is finite is $\boxed{-\pi <x<\pi }$; the apparent exclusion of $0$ arose only from division by $\sin x$. This is continuation through a zero of the leading ODE coefficient.