Crazy solve

Suppose the integral solutions of the equation : x 3 + 10 x 1 = y 3 + 6 y 2 x^{3}+10x-1=y^{3}+6y^{2} are

( x 1 , y 1 ) , , ( x n , y n ) (x_{1},y_{1}),\ldots,(x_{n},y_{n}) .

Find the value of ( x 1 + y 1 ) + + ( x n + y n ) (x_{1}+y_{1})+\ldots+(x_{n}+y_{n}) .


The answer is 10.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Kazem Sepehrinia
Jul 17, 2015

y 2 y^2 term on the RHS of the equation tells us that y y can not be greater than x x . We'll find out that, let x y = z x-y=z ( y + z ) 3 + 10 ( y + z ) 1 = y 3 + 6 y 2 ( 3 z 6 ) y 2 + ( 3 z 2 + 10 ) y + z 3 + 10 z 1 = 0 (y+z)^3+10(y+z)-1=y^3+6y^2 \\ (3z-6)y^2+(3z^2+10)y+z^3+10z-1=0 Discriminant of the obtained quadratic (in terms of y y ) must be non-negative (why?) ( 3 z 2 + 10 ) 2 4 ( 3 z 6 ) ( z 3 + 10 z 1 ) 0 0 z 6 (3z^2+10)^2-4(3z-6)(z^3+10z-1)\ge0 \\ 0\le z\le6 Trying these values gives y = 5 y=5 for z = 1 z=1 and y = 3 y=-3 for z = 5 z=5 only. So totally two pairs ( x , y ) = ( 6 , 5 ) , ( 2 , 3 ) (x, y)=(6, 5), (2, -3)

Great solution, understandable and intuitive.

Garrett Clarke - 5 years, 11 months ago

Log in to reply

Thanks Garrett :)

Kazem Sepehrinia - 5 years, 10 months ago

H o w d o w e t r y 0 x y 6 a n d w h y t h e r e c o u l d n o t b e m o r e p a i r s ? x = 100 , y = 101 s a t i s f i e s 0 x y 6. s o a l s o x = 101 , y = 100. How~ do~ we~ try~0\leq x-y\leq 6~and~ why ~there~ could~ not~ be ~more~ pairs? \\x=-100, y=-101~satisfies~0\leq x-y\leq 6.~~so ~also~x=101, ~y=100.

Niranjan Khanderia - 5 years, 10 months ago

We may program on spread-sheet, calculator, or a computer language:- I n i t i a l B = C = 100 O u t e r D o l o o p A = 100 , 1 , 100 I n n e r D o L o o p x = 100 , 1 , 100 C = B B = x 3 + 10 x 1 A + 6 A 2 I f B = 0 P r i n t x a n d A I f C B 0 e n d i n n e r l o o p g o t o n e x t f o r o u t t e r l o o p . For calculator with the help of ENTRY in TI calculators, and STO things like Printing, ending loop, going to next, If, all are manual. We remain satisfied investigating between -100 and 100!!. \text{We may program on spread-sheet, calculator, or a computer language:-}\\Initial~~ B=C=100\\ Outer ~Do ~loop~ A=-100,1,100\\Inner~Do~Loop~x=-100,1,100\\ C=B\\\large B=\dfrac{x^3+10x-1}{A+6}-A^2 \\ If ~B=0~~ Print ~~x~and~A \\ If~C*B\leq 0~end~inner~loop~go~to~next~for~outter~loop. \\ \text{For calculator with the help of ENTRY in TI calculators, and STO} \\ \text{ things like Printing, ending loop, going to next, If, all are manual. } \\\text{We remain satisfied investigating between -100 and 100!!.}

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...