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Integral of 4/x

Posted on 2011-03-07
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I understand that the integral of 1/x is ln x + c where x > 0
but what happens if you integrate 4/x?

Is it 4.ln x + c?

I'm not sure where the 4 fits in to the answer.
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Question by:purplesoup
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akajohn earned 1200 total points
ID: 35055352
I hope I can make this clear

4 * 1/x

Integrate and you get


4 ln x + k

Here my constant K is arbirtrary and is equal to 4*c.

4 times a constant is just another constant.

Does that help?
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by:deighton
deighton earned 400 total points
ID: 35056040
an example is say, integrate 1 / (2x)

by substituting
u = 2x
du = 2dx
dx = du/2

you can get

(1/2) ln(2x) + c

if you differentiate that, you get

(1/2) 2 / 2x = 1/(2x)

however, note that

(1/2) ln(2x) + c = (1/2) (ln(x) + ln(1/2)) + c = (1/2) ln(x) + (1/2) ln(1/2) + c

=  (1/2) ln(x) + k

absorbing the term into another arbitrary constant in that case.

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by:phoffric
phoffric earned 400 total points
ID: 35056197
In general, for any constant b:
     § b f(x) dx = b § f(x) dx
i.e., you can bring the constant factor outside of the integral.

You may have already seen that the integral can be used to calculate the area under a curve. So, if you calculated the area under the function f(x) in some interval, then if you consider another function g(x) = 2 f(x), then g(x) is twice the value of f(x) for any particular x. Then you would expect that the area under the curve g(x) to be twice as much as the area under the curve f(x).

For your indefinite integra, if
     § f(x) dx = h(x) +c
then from above, you would say
     § b f(x) dx = b § f(x) dx = b( h(x) +c ) = b h(x) + b c
where h(x) is just the integral of f(x)

But since b and c are both constants, you can just treat them as a single constant, call it C.
     § b f(x) dx = b h(x) + C


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