Let the first order differential equation be d y <mrow class="MJX-TeXAtom-ORD"> /

Ezekiel Yoder

Ezekiel Yoder

Answered question

2022-06-12

Let the first order differential equation be
d y / d x + P ( x ) y = Q ( x )
To solve this we multiply it by function say v(x) then it becomes
v ( x ) d y / d x + P ( x ) v ( x ) y = Q ( x ) v ( x )
d ( v ( x ) y ) / d x = Q ( x ) v ( x )
where d ( v ( x ) ) / d x = P ( x ) v ( x )
I don't write full solution here but now here to obtain integrating factor v(x) from above equation we get l n | v ( x ) | = P ( x ) d x.
Now my question is why we neglect absolute value of v(x) and obtain integrating factor as e P ( x ) d x = v ( x )

Answer & Explanation

seraphinod

seraphinod

Beginner2022-06-13Added 22 answers

v can not change its sign, because ln | v ( x ) | always has a finite value. For the integrating factor, you just need one of the solutions of the homogeneous equation, so choosing the symbolically minimal v ( x ) = e 0 x P ( s ) d s is sufficient.

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