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find b - a

Albert

Well-known member
Jan 25, 2013
1,225
$a,b\in R$

$if :\,\,5a^2+8ab+5b^2+170=50a+58b$

please find :$b-a$
 

mente oscura

Well-known member
Nov 29, 2013
172
Re: find b-a

$a,b\in R$

$if :\,\,5a^2+8ab+5b^2+170=50a+58b$

please find :$b-a$
Hello.

[tex]5a^2-a(50-8b)+5b^2-58b+170=0[/tex]

[tex]a=\dfrac{50-8b \pm \sqrt{-36b^2-360b-900}}{10}[/tex]

[tex]b=-5[/tex]

[tex]\forall{b}>-5 \ and\ \forall{b}<-5 \rightarrow{b \cancel{\in{R}}}[/tex]

[tex]If \ b=-5 \rightarrow{a \cancel{\in{R}}}[/tex]

Conclusion:

[tex]\cancel{\exists}{a,b} \in{R} \ / \ 5a^2+8ab+5b^2+170=50a+58b[/tex]


Regards.
 

mathbalarka

Well-known member
MHB Math Helper
Mar 22, 2013
573
Re: find b-a

Untrue. A doable solution is :

(1, 5)
 

mathbalarka

Well-known member
MHB Math Helper
Mar 22, 2013
573
Re: find b-a

I don't usually post solutions to elementary number theory, but doing so to point out mente oscura's flaw :

Going in the line of mente oscura, we have :

$$5a^2-a(50-8b)+5b^2-58b+170=0$$

which has the discriminant of $-36b^2+360b-900 = 36(5-b)^2$

This easily gives $b = 5$
 

mente oscura

Well-known member
Nov 29, 2013
172

Albert

Well-known member
Jan 25, 2013
1,225
Re: find b-a

$a,b\in R$

$if :\,\,5a^2+8ab+5b^2+170=50a+58b$

please find :$b-a$
solution:

$(2a+b)^2+(2b+a)^2+170=50a+58b---(1)$
let :$x=2a+b,\,\, y=(2b+a)$
then :$a=\dfrac{2x-y}{3},\,\, b=\dfrac{2y-x}{3}$
(1)becomes:$3(x-7)^2+3(y-11)^2=0$
we have :$x=7,\,\, y=11$
$\therefore y-x=b-a=4$
 
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