THE USE OF INTEGRATING FACTORS:
THIS DISCUSSION IS MOTIVATED BY THE
RECOGNITION THAT d/dx(yeax) = ayeax + eax = eax(ay + dy/dx)
THIS PATTERN SUGGESTS THAT THE
DIFFERENTIAL EQUATION dy/dx + ay = f(x)
CAN BE APPROACHED BY MULTIPLYING BY
THE INTEGRATING FACTOR eax
eax(dy/dx) + eax y = eax f(x)
INTEGRATION OF THIS PROCEEDS BY USING THE ABOVE