Which of these is a second-order linear ODE?
y′′+3y′+2y=0y'' + 3y' + 2y = 0y′′+3y′+2y=0
y′+y=xy' + y = xy′+y=x
y′′+y2=0y'' + y^2 = 0y′′+y2=0
(y′′)2=x(y'')^2 = x(y′′)2=x