Consider the integral, I=∫1xdx\displaystyle I =\int \dfrac{1}{x}dx I=∫x1dx
We know that I=logx I= \log x I=logx
But, I=∫1x(1)dx=(x)1x−∫(−1x2)xdx\displaystyle I =\int \dfrac{1}{x}(1)dx =(x)\dfrac{1}{x}-\int \bigg(-\dfrac{1}{x^2} \bigg) xdxI=∫x1(1)dx=(x)x1−∫(−x21)xdx
1+I=I⇒1=01+I=I \Rightarrow 1=01+I=I⇒1=0
What is the mistake?
I do not know the mistake
Note by Sparsh Sarode 4 years, 5 months ago
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you actually got ∫1x−∫1x=1\int \frac{1}{x} -\int \frac{1}{x}=1 ∫x1−∫x1=1. There is absolutely nothing wrong with that. Those are indefinite integrals.
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What if I add limits from 1 to 2..you still get 1=0
I should have added limits.
It is xx∣ab\frac{x}{x} |_{a}^{b}xx∣ab. That yeilds zero. 1 there has limits.
@Kushal Patankar – I didn't quite get u, u meant xx=b−ab−a≠1 \dfrac{x}{x} =\dfrac{b-a}{b-a} \neq 1xx=b−ab−a=1
@Sparsh Sarode – I can't believe you just stated that...
@Kushal Patankar – Correct me if I am wrong, isn't 2−12−1=11=1 \dfrac{2-1}{2-1}=\dfrac{1}{1}=12−12−1=11=1 ?
@Sparsh Sarode – its bb−aa\frac{b}{b}-\frac{a}{a}bb−aa and still 1∣ab=1a−1b=1−1=01|_a^b = 1_a-1_b=1-1=01∣ab=1a−1b=1−1=0.
@Kushal Patankar – Oops! I can't believe I made such a mistake... Anyways thx..
@Sparsh Sarode – Sure... Glad to help.
@Sparsh Sarode – Can you post this as a problem so others can learn from this misconception?
@Calvin Lin – Ok, surely
@Kushal Patankar – Exactly... Then he should get correctly.
How is LHS not equal to 0
Integration constants need not be equal.
@Kushal Patankar – K.. I agree on that
And what happens without limits? ∫dxx=xx+∫dxx\displaystyle \int \frac{dx}{x} =\frac{x}{x}+\displaystyle \int \frac{dx}{x}∫xdx=xx+∫xdx
It's still correct.. Integration constants need not be same
Oh, right, thank you
@Hjalmar Orellana Soto – Welcome.. :)
You cannot cancel the integrals on both sides because each function has infinitely many anti-derivatives off by a constant. That's why, for good reason, you should always include "+C" in your answers.
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Comments
you actually got ∫x1−∫x1=1. There is absolutely nothing wrong with that. Those are indefinite integrals.
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What if I add limits from 1 to 2..you still get 1=0
I should have added limits.
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It is xx∣ab. That yeilds zero. 1 there has limits.
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xx=b−ab−a=1
I didn't quite get u, u meantLog in to reply
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2−12−1=11=1 ?
Correct me if I am wrong, isn'tLog in to reply
bb−aa and still 1∣ab=1a−1b=1−1=0.
itsLog in to reply
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How is LHS not equal to 0
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Integration constants need not be equal.
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And what happens without limits? ∫xdx=xx+∫xdx
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It's still correct.. Integration constants need not be same
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Oh, right, thank you
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You cannot cancel the integrals on both sides because each function has infinitely many anti-derivatives off by a constant. That's why, for good reason, you should always include "+C" in your answers.