Quaint quintic

Algebra Level 3

The polynomial expression in two variables

f ( x , y ) = x 5 + 3 y 2 3 x 2 + y 5 f(x,y) = x^5 + 3y^2 - 3x^2+y^5

can be factorized into the product of two non-constant polynomials with integer coefficients, namely

f ( x , y ) = g ( x , y ) × h ( x , y ) . f(x,y) = g(x,y) \times h(x,y).

Evaluate g ( 2 , 2 ) + h ( 2 , 2 ) . |g(2,2)|+|h(2,2)|.


The answer is 20.

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12 solutions

Steven Hao
Oct 20, 2013

x + y f ( x , y ) g ( x , y ) = ( x + y ) h ( x , y ) = f ( x + y ) g ( x + y ) g ( 2 , 2 ) = 4 , f ( 2 , 2 ) = 64 h ( 2 , 2 ) = 16 g ( 2 , 2 ) + h ( 2 , 2 ) = 20 x+y|f(x,y)\\ \implies g(x,y)=(x+y)\\\implies h(x,y)=\frac{f(x+y)}{g(x+y)}\\ g(2,2)=4, f(2,2) = 64\implies h(2,2) = 16\\\implies |g(2,2)|+|h(2,2)|=\boxed{20}

Nice! Sweet and simple.

Timothy Zhou - 7 years, 7 months ago

Simple but awesome !!!!

Albert Pranata - 7 years, 7 months ago

Can someone explain this step by step? I'm having a lot of problems understanding this. Thanks!

Joseph Izaguirre - 7 years, 7 months ago

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I made some typos: in particular, the third line should read h ( x , y ) = f ( x , y ) g ( x , y ) h(x,y)=\frac{f(x,y)}{g(x,y)} . You can substitute in the expressions for f f and g g and find the exact polynomial h h , but I omitted that step because it's not necessary to evaluate h h at a single point.

Once you find g g and h h , the two factors of f f , finding the final answer is only a matter of plugging in ( 2 , 2 ) (2,2) and evaluating the desired expression.

Steven Hao - 7 years, 7 months ago

Brilliant

P. Sundar Raman. - 7 years, 7 months ago

WOW

Opel Berlin - 7 years, 7 months ago

How did you know that g(x,y) = (x+y)?

Edward Keller - 7 years, 7 months ago

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Because x + y x 5 + y 5 x+y|x^5+y^5 and x + y x 2 y 2 x+y|x^2-y^2

Steven Hao - 7 years, 7 months ago

or you can just 'guess' in this special case because of the symmetry, that f(x,y)=0 when x=1 and y=-1. From the remainder/factor theorem, this means that x-1+y-(-1) must be a factor i.e. x+y is a factor. Not so elegant but still a method of getting there.

Victor Phan - 7 years, 7 months ago

Because the polynomial is 0 when x = -y

Mark Gordon - 7 years, 7 months ago

it's very confusing!right?

Dexter Paul De Jesus - 7 years, 7 months ago

y=3x+2 change it to standard form

Moises Llamas - 7 years, 7 months ago
Jubayer Nirjhor
Oct 20, 2013

Consider f ( x ) = x 5 + 3 y 2 3 x 2 + y 5 f ( y ) = 0 f(x) = x^5+3y^2-3x^2+y^5 ~~~~~~~~~ \therefore f(-y)=0

By the factor theorem, we know that ( x + y ) (x+y) is a factor of f ( x ) f(x) ...

Further factorizing f ( x ) f(x) , we get... f ( x , y ) = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 3 x + 3 y ) f(x,y)=(x+y) (x^4-x^3 y+x^2 y^2-x y^3+y^4-3x+3 y)

Since, this factorization is irreducible, hence, g ( x , y ) g(x,y) and h ( x , y ) h(x,y) must be ( x + y ) (x+y) and ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 3 x + 3 y ) (x^4-x^3 y+x^2 y^2-x y^3+y^4-3x+3 y) in some order. The order doesn't matter since the problem requires the answer for same values of x x and y y . Simply plugging in x = 2 x=2 and y = 2 y=2 in both equation, we get...

g ( 2 , 2 ) = 4 = 4 |g(2,2)|=|4|=4 and h ( 2 , 2 ) = 16 = 16 |h(2,2)|=|16|=16

Therefore, g ( 2 , 2 ) + h ( 2 , 2 ) = 4 + 16 = 20 |g(2,2)|+|h(2,2)|=4+16=\fbox{20}

this is also my solution.

Dexter Paul De Jesus - 7 years, 7 months ago

Good use of the technique Remainder-Factor Theorem , applied to 2 variables.

Calvin Lin Staff - 7 years, 7 months ago
Daniel Ferreira
Oct 22, 2013

De x n + y n = ( x + y ) ( x n 1 y 0 x n 2 y 1 + x n 3 y 3 . . . x 1 y n 2 + x 0 y n 1 ) x^n + y^n = (x + y)(x^{n - 1} \cdot y^0 - x^{n - 2} \cdot y^1 + x^{n - 3} \cdot y^3 - ... - x^1 \cdot y^{n - 2} + x^0 \cdot y^{n - 1}) ,

Temos que x 5 + y 5 = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 ) \boxed{x^5 + y^5 = (x+y)(x^4 - x^3y + x^2y^2 - xy^3 + y^4)}

Segue que,

f ( x , y ) = y 5 + x 5 + 3 y 2 3 x 2 f ( x , y ) = ( y 5 + x 5 ) + 3 ( y 2 x 2 ) f ( x , y ) = ( y + x ) ( y 4 y 3 x + y 2 x 2 y x 3 + x 4 ) + 3 ( y + x ) ( y x ) f ( x , y ) = ( y + x ) ( y 4 y 3 x + y 2 x 2 y x 3 + x 4 + 3 y 3 x ) { g ( x , y ) = x + y h ( x , y ) = y 4 y 3 x + y 2 x 2 y x 3 + x 4 + 3 y 3 x f(x, y) = y^5 + x^5 + 3y^2 - 3x^2 \\\\ f(x, y) = (y^5 + x^5) + 3(y^2 - x^2) \\\\ f(x, y) = (y + x)(y^4 - y^3x + y^2x^2 - yx^3 + x^4) + 3(y + x)(y - x) \\\\ f(x, y) = (y + x)(y^4 - y^3x + y^2x^2 - yx^3 + x^4 + 3y - 3x) \\\\ \begin{cases} g(x, y) = x + y \\ h(x, y) = y^4 - y^3x + y^2x^2 - yx^3 + x^4 + 3y - 3x \end{cases}

Por conseguinte,

g ( x , y ) = x + y g ( 2 , 2 ) = 2 + 2 g ( 2 , 2 ) = 4 g ( 2 , 2 ) = 4 g(x, y) = x + y \\ g(2, 2) = 2 + 2 \\ g(2, 2) = 4 \\ \boxed{|g(2, 2)| = 4}

E,

h ( x , y ) = y 4 y 3 x + y 2 x 2 y x 3 + x 4 + 3 y 3 x h ( 2 , 2 ) = 16 16 + 16 16 + 16 + 6 6 h ( 2 , 2 ) = 16 h ( 2 , 2 ) = 16 h(x, y) = y^4 - y^3x + y^2x^2 - yx^3 + x^4 + 3y - 3x \\ h(2, 2) = 16 - 16 + 16 - 16 + 16 + 6 - 6 \\ h(2, 2) = 16 \\ \boxed{|h(2, 2)| = 16}

Por fim, g ( 2 , 2 ) + h ( 2 , 2 ) = 20 \boxed{\boxed{|g(2, 2)| + |h(2, 2)| = 20}}

Thaddeus Abiy
Oct 21, 2013

Factoring out x 5 + y 5 x^5+y^5 we have:

x 5 + y 5 = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 ) x^5+y^5=(x+y)(x^4-x^{3}y+x^{2}y^2-xy^{3}+y^4)

x 5 + y 5 + 3 y 2 3 x 2 = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 ) + 3 ( y x ) ( x + y ) x^5+y^5+3y^2-3x^2 = (x+y)(x^4-x^{3}y+x^{2}y^2-xy^{3}+y^4) + 3(y-x)(x+y)

Factoring for ( y + x ) (y+x)

x 5 + y 5 + 3 y 2 3 x 2 = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 + 3 ( y x ) x^5+y^5+3y^2-3x^2 =(x+y)(x^4-x^{3}y+x^{2}y^2-xy^{3}+y^4+3(y-x)

Thus we have

f ( x , y ) = x + y f(x,y)=x+y and g ( x , y ) = x 4 x 3 y + x 2 y 2 x y 3 + y 4 + 3 ( y x ) g(x,y)=x^4-x^{3}y+x^{2}y^2-xy^{3}+y^4+3(y-x)

f ( 2 , 2 ) + g ( 2 , 2 ) = 4 + 16 = 20 f(2,2)+g(2,2)=4+16=20

Michael Tang
Oct 23, 2013

Notice that we can write

f ( x , y ) = x 5 + 3 y 2 3 x 2 + y 5 = ( x 5 + y 5 ) ( 3 x 2 3 y 2 ) = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 ) ( x + y ) ( 3 x 3 y ) = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 + 3 x 3 y ) \begin{aligned} f(x,y) &= x^5 + 3y^2 - 3x^2 + y^5 \\ &= (x^5+y^5) - (3x^2-3y^2) \\ &= (x+y)(x^4-x^3y+x^2y^2-xy^3+y^4) \\ &\qquad \qquad\qquad\qquad - (x+y)(3x-3y) \\ &= (x+y)(x^4-x^3y+x^2y^2-xy^3+y^4+3x-3y) \end{aligned}

[It remains to prove that the second factor cannot be factorized further.]

Therefore, g ( x , y ) = x + y g(x,y) = x+y and h ( x , y ) = x 4 x 3 y + x 2 y 2 x y 3 + y 4 + 3 x 3 y , h(x,y) = x^4-x^3y+x^2y^2-xy^3+y^4+3x-3y, so we have g ( 2 , 2 ) + h ( 2 , 2 ) = 4 + 16 = 20 . |g(2,2)| + |h(2,2)| = 4 + 16 = \boxed{20}.

Wahyu Benardo
Oct 20, 2013

hint : root of f(x,y) is 0,0 then, f(x,y) is divisible by xy. g(x,y) = xy h(x,y) = f(x,y)/g(x,y)

evaluate the question at point (2,2)

That only implies that (0,0) is the root of one of its factors. Take, for example, f ( x , y ) = x y f(x,y) = x - y . (0,0) is a root of f, but xy is not a factor of f.

Steven Hao - 7 years, 7 months ago

Rewrite f ( x , y ) f(x,y) as

( x 5 + y 5 ) + ( 3 y 2 3 x 2 ) (x^5 + y^5) + (3y^2 - 3x^2)

( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 ) + 3 ( y x ) ( y + x ) (x+y)(x^4 - x^3y + x^2y^2 - xy^3 + y^4) + 3(y-x)(y+x)

( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 + 3 y 3 x ) ) (x+y)(x^4 - x^3y + x^2y^2 - xy^3 + y^4 + 3y-3x))

Therefore g ( x , y ) = ( x + y ) g(x,y) = (x+y) and h ( x , y ) = ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 + 3 y 3 x ) h(x,y) = (x^4 - x^3y + x^2y^2 - xy^3 + y^4 + 3y - 3x)

And g ( 2 , 2 ) + h ( 2 , 2 ) = ( 2 + 2 ) + ( 2 4 2 3 2 + 2 2 2 2 2 2 3 + 2 4 + 3 2 3 2 ) |g(2,2)| + |h(2,2)| = |(2+2)| + |(2^4 - 2^3*2 + 2^2*2^2 -2*2^3 + 2^4 + 3*2 - 3*2)|

= 4 + 16 = 4 + 16 = 20 = |4| + |16| = 4+16 = \boxed{20}

Nicole Moore
Oct 25, 2013

Input this into Wolfram Alpha:
* x 5 3 x 2 + 3 y 2 + y 5 x^{5} - 3x^{2} + 3y^{2} + y^{5} *

In alternative forms, you should see this:

( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 3 x + y 4 + 3 y ) (x+y)(x^{4}-x^{3}y+x^{2}y^{2}-xy^{3}-3x+y^{4}+3y)

Replace x and y with 2, as shown below,

( 2 + 2 ) ( 2 4 2 3 2 + 2 2 2 2 2 × 2 3 3 × 2 + 2 4 + 3 × 2 ) (2+2)(2^{4}-2^{3}2+2^{2}2^2-2\times2^{3}- 3 \times2+2^{4}+3\times2)

then solve 4 + 16 16 + 16 16 6 + 16 + 6 |4|+|16-16+16-16-6+16+6|

Notice the 6's cancel out, as do all but one 16

Now add the remaining numbers 4 + 16 |4|+|16|

The answer is 20

Dhurjati Das
Oct 26, 2013

x^5+y^5=(x+y)(x^4-x^3y+x^2y^2-xy^3+y^4) 3(y^2-x^2)=3(y-x)(y+x)

Yash Mittal
Oct 24, 2013

on observing the eq., it is clear that x=-y is one sol. hence a factor: g(x,y)=(x+y). now, g(2,2)=4 therefore, h(2,2)=f(2,2)/g(2,2)=16 so that h(2,2)+g(2,2)=20

f ( x , y ) = x 5 + y 5 3 ( x 2 y 2 ) f(x,y)=x^5+y^5-3(x^2-y^2)
f ( x , y ) = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 ) 3 ( x + y ) ( x y ) f(x,y)=(x+y)(x^4 - x^3y + x^2y^2 - xy^3 + y^4) - 3(x+y)(x-y)
f ( x , y ) = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 + y 4 3 x + 3 y ) f(x,y) = (x+y)(x^4 - x^3y + x^2y^2 - xy^3 + y^4 - 3x + 3y)
Set g ( x , y ) = x + y g(x,y)=x+y and h ( x , y ) = x 4 x 3 y + x 2 y 2 x y 3 + y 4 3 x + 3 y h(x,y)=x^4 - x^3y + x^2y^2 - xy^3 + y^4 - 3x + 3y
g ( 2 , 2 ) = 2 + 2 = 4 g(2,2) = 2+2=4
h ( 2 , 2 ) = 2 4 2 3 × 2 + 2 2 × 2 2 2 × 2 3 + 2 4 3 ( 2 ) + 3 ( 2 ) h(2,2) = 2^4 - 2^3\times 2 + 2^2 \times 2^2 - 2 \times 2^3 + 2^4 - 3(2)+3(2)
h ( 2 , 2 ) = 16 h(2,2) = 16
g ( 2 , 2 ) + h ( 2 , 2 ) = 4 + 16 = 20 |g(2,2)|+|h(2,2)|=4+16=20






Jan J.
Oct 21, 2013

Note that f ( k , k ) = 0 f(k,-k) = 0 , hence x + y x + y is a factor by the factor theorem. By the polynomial division we get x 5 + 3 y 2 3 x 2 + y 5 x + y = x 4 x 3 y + x 2 y 2 x y 3 3 x + y 4 + 3 y \frac{x^5 + 3y^2 - 3x^2 + y^5}{x + y} = x^4-x^3 y+x^2 y^2-x y^3-3 x+y^4+3 y Hence f ( x , y ) = ( x + y ) ( x 4 x 3 y + x 2 y 2 x y 3 3 x + y 4 + 3 y ) f(x,y) = (x + y)(x^4-x^3 y+x^2 y^2-x y^3-3 x+y^4+3 y) So g ( 2 , 2 ) + h ( 2 , 2 ) = 20 |g(2,2)| + |h(2,2)| = \boxed{20}

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