Hey ,in the 5th step ,from where has -1 arrived in the second bracket
I mean when you used the formula $a^{3} - b^{3}$
the correct equation should have been this
$(a^{2} + b^{2} + 2ab +1 +a+b$)

I plotted $f(a) = a^3+b^3+3ab-1$ for $b = -10, -7.5, 0, 7.5, 10$, thinking that there will be infinite solutions. But I discovered that the roots to $f(a)$ are respectively (a=11,8.5,1,-6.5,-9) making $\boxed{a+b = 1}$. See the graph here.

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## Comments

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TopNewestSo , here is my trial:

$a^3+b^3+3ab-1=0 \\ (a+b)^3-3ab(a+b)+3ab-1=0 \\ (a+b)^3 - 3ab(a+b-1) -1=0 \\ (a+b)^3 - 1^3 = 3ab(a+b-1) \\ (a+b-1)(a^2+b^2+2ab+a+b+1)= 3ab(a+b-1)$

Let $a+b-1 \neq 0$

$\Rightarrow a^2+b^2+2ab+a+b+1= 3ab \\ a^2+b^2-ab+a+b+1=0 \\ a^2+b^2-(a-1)(b-1)=0 \\ a^2+b^2=(a-1)(b-1) \\ (a+b)^2 - 2ab = (a-1)(b-1) \\ (a+b+\sqrt{2ab})(a+b-\sqrt{2ab})=(a-1)(b-1) \\ \Rightarrow b+\sqrt{2ab} = -1 \quad a-\sqrt{2ab}=-1 \\ \Rightarrow a+b=-2 \quad a+b=1$

Is it right?

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yup, it's right

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Do like and reshare this for others (not for my sake). Thanks!

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Note @Mehul Arora @Vaibhav Prasad , $a,b$ are real numbers and not just integers. Cheers!

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Hey ,in the 5th step ,from where has

-1arrived in the second bracket I mean when you used the formula $a^{3} - b^{3}$ the correct equation should have been this$(a^{2} + b^{2} + 2ab +1 +a+b$)Log in to reply

Sorry , it was a typing mistake. Thanks for correcting me.

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Please explain the second last step

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$(a+b+\sqrt{2ab})(a+b-\sqrt{2ab})=(a-1)(b-1) \\ \Rightarrow (a+(b+\sqrt{2ab}))(b+(a-\sqrt{2ab}))=(a-1)(b-1)$

If you compare the factors of $L.H.S$ and $R.H.S$ , correspondingly you will get:

$\Rightarrow b+\sqrt{2ab} = -1 \quad a-\sqrt{2ab}=-1$

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$\displaystyle a^3 + b^3 + c^3 -3abc = \dfrac{1}{2} (a+b+c)( (a-b)^2 + (b-c)^2 + (c-a)^2)$

Now substitute $c = -1$ and then L.H.S equals 0

$\displaystyle 0 = (a+b-1)( (a-b)^2 + (a+1)^2 + (b+1)^2)$

From here

$a+b = 1$ from the first and

$a+b = -2$ from second (easily solvable)

EDIT :- the second equation has only one solution when $a =-1$.

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Nice , that crucial substitution really helped!

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That identity is very useful it is just a result derived from another identity.

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The answer is $\space \boxed{a+b=1}$.

I plotted $f(a) = a^3+b^3+3ab-1$ for $b = -10, -7.5, 0, 7.5, 10$, thinking that there will be infinite solutions. But I discovered that the roots to $f(a)$ are respectively (a=11,8.5,1,-6.5,-9) making $\boxed{a+b = 1}$. See the graph here.

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Sir , note that $a+b=-2$ is also a solution. See my and Krishna's solution. Thanks!

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There is one special case when $a+b=-2$.

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@Calvin Lin @siddharth bhatt @Pi Han Goh @Brian Charlesworth @Curtis Clement @Azhaghu Roopesh M @Mehul Arora @Chew-Seong Cheong @Prasun Biswas @everyone no need of help now.

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What do you need help with? You solved the problem didn't you? :)

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Actually , I got it just after I posted this note. Since you have nice methods and approaches , I mentioned you.

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1 :- The answer

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I am not asking for the answer. I am asking for an elegant accurate solution with proof. -_-

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How about $a=-1$ and $b=-1$

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I am not asking for the answer. I am asking for an elegant accurate solution with proof. -_-

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See my bash , no need to compute $a,b$ :)

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Let a+b=x Now If, x=1, Then the Equation becomes a+b ^ 3 Thena+b=1 :D

P.S. I'm just too lazy. Idk What I am typing right now.

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That's right.

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I know.

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$a,b$ :)

See my bash , no need to computeLog in to reply

a+b can be 1 also

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Yeah it can be $1$. So?

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LOL xD

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