1. y ' P (x ) ⋅ y = r (x )
+
h = ∫ P ( x ) dx = ∫ k ⋅ dx + C = kx + C
e − h = e − kx

⇒  h ⇒ y = e − h ⋅  ∫ e h ⋅ r ( x ) ⋅ dx + C 
e = e
kx  

⇒ y = e − kx  ∫ e kx ⋅ e 2 kx dx + C 
( )
 
1
= e − kx  ∫ e 3kx dx + C  =
( ) ⋅ e − kx + 3kx + C ⋅ e − kx
  3k
1
= ⋅ e 2 kx + C ⋅ e − kx ~~~ GS (if k ≠ 0)
3k