# Importance of adding the constant of integration.

by shayaan_musta
 P: 184 Hello experts! I have a question below. Why is it important to add the constant of integration immediately when the integration is performed? Thanks in advance.
 P: 926 If you're wondering why there is a constant of integration, it is because when you have a function f(x) such that F(x) = f'(x), then the derivative of g(x) = f(x) + C for any constant also equals F(x) (i.e., g'(x) = f'(x)), so when you integrate F(x), you need to capture that constant in the solution. Now, if you are asking why it has to be done immediately I'm not sure what you're asking since adding the constant is done as the last step.
 Sci Advisor HW Helper P: 11,896 To the OP: Assume you must do a double integration. For example: $$\frac{d^2 f(x)}{d x^2} = x^3 + 5$$ What is then f(x) equal to ?
P: 184
Importance of adding the constant of integration.

 Quote by dextercioby To the OP: Assume you must do a double integration. For example: $$\frac{d^2 f(x)}{d x^2} = x^3 + 5$$ What is then f(x) equal to ?
f(x)=$\frac{x^{5}}{5}$ + $\frac{5x^{2}}{2}$ + c

where "c" is the integration constant.

Now what?? :s
P: 184
 Quote by daveb If you're wondering why there is a constant of integration, it is because when you have a function f(x) such that F(x) = f'(x), then the derivative of g(x) = f(x) + C for any constant also equals F(x) (i.e., g'(x) = f'(x)), so when you integrate F(x), you need to capture that constant in the solution. Now, if you are asking why it has to be done immediately I'm not sure what you're asking since adding the constant is done as the last step.
Ok daveb thank you. I think you have answered my question.
Thank you very much.
 P: 184 Thank you dextercioby. You also made me to think the answer of my question.
P: 24
 Quote by shayaan_musta f(x)=$\frac{x^{5}}{5}$ + $\frac{5x^{2}}{2}$ + c where "c" is the integration constant. Now what?? :s
Not quite, this is why the constant of integration is important. The first integration gives

$$\frac{df}{dx}=\frac{x^4}{4}+5x+c_1$$

and the second integration gives

$$f(x)=\frac{x^5}{20}+\frac{5x^2}{2}+c_1x+c_2$$

Which is why we need one constant of integration for each integral
 Math Emeritus Sci Advisor Thanks PF Gold P: 39,345 Note that dextercioby's example is a linear second order non-homogenous differential equation which means that the set of all solutions is a two dimensional "linear manifold". That is why you need two undetermined coefficients.
P: 184
 Quote by cpt_carrot Not quite, this is why the constant of integration is important. The first integration gives $$\frac{df}{dx}=\frac{x^4}{4}+5x+c_1$$ and the second integration gives $$f(x)=\frac{x^5}{20}+\frac{5x^2}{2}+c_1x+c_2$$ Which is why we need one constant of integration for each integral
Ok cpt_carrot I got you. Thanks man.
Now I have understood that for each integration a constant is important, as Hallsofivy said.

So help guys on Physics Forums

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