y'+a*y=b

                                             y ' + a·y = b                                     multiplicer med e^ax

                                             e^ax·y ' + e^ax·a·y = b·e^ax

                                             (e^ax·y) ' = b·e^ax                               integreres med hensyn til x

                                             e^ax·y  = ∫b·e^axdx

                                             e^ax·y = (b/a)·e^ax  + C                      divideres med  e^ax
 

                                             y = C·e^-ax + (b/a)

kontrolberegning
                               
                             y ' = -a·C·e^-ax

                             y ' = -a(y-(b/a))

                             y ' = -ay+b

                             y '+ay = b