Why is the equation $dy/dx + P(x)y=Q(x)$ said to be standard form? Well, I know that in linear differential equation the variable and its derivatives are raised to power of $1$ or $0$. But I am confused where did the standard form of linear differential equation came form?
That is, why is the equation $dy/dx + P(x)y=Q(x)$ said to be standard form? 

The standard form of a differential equation is 
‎$$f_n\frac{d^ny}{dx^n}+f_{n-1}\frac{d^{n-1}y}{dx^{n-1}}+\cdots+f_1\frac{dy}{dx}+f_0y=g$$‎
for $n=1$, this called differential equation of the first order, i.e.
‎$$f_1\frac{dy}{dx}+f_0y=g$$‎
you take
‎$$\frac{dy}{dx}+py=q$$‎
while $p$ and $q$ are continuous functions of $x$, this called linear differential equation of the first order.
