Difference between revisions of "Sum and difference of powers"

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The '''sum and difference of powers''' are powerful factoring techniques that, respectively, factor a sum or a difference of certain powers.
  
The '''Sum and Difference of Cubes''' identities refer to two powerful factoring techniques that, respectively, factor a sum of cubes and a difference of cubes.
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==Sums of Odd Powers==
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<math>a^{2n+1}+b^{2n+1}=(a+b)(a^{2n}-a^{2n-1}b+a^{2n-2}b^2-\ldots-ab^{2n-1}+b^{2n})</math>
  
==Factored Forms of Sums and Differences of Cubes==
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==Differences of Powers==
* <math> a^3 + b^3 = (a+b)(a^2-ab+b^2)</math>
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If <math>p</math> is a positive integer and <math>x</math> and <math>y</math> are real numbers,
* <math>a^3-b^3 = (a-b)(a^2+ab+b^2)</math>
 
  
==See also==
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<math>x^{p+1}-y^{p+1}=(x-y)(x^p+x^{p-1}y+\cdots +xy^{p-1}+y^p)</math>
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For example:
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<math>x^2-y^2=(x-y)(x+y)</math>
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<math>x^3-y^3=(x-y)(x^2+xy+y^2)</math>
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<math>x^4-y^4=(x-y)(x^3+x^2y+xy^2+y^3)</math>
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Note that the number of terms in the ''long'' factor is equal to the exponent in the expression being factored.
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An amazing thing happens when <math>x</math> and <math>y</math> differ by <math>1</math>, say, <math>x = y+1</math>. Then <math>x-y = 1</math> and
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<math>x^{p+1}-y^{p+1}=(y+1)^{p+1}-y^{p+1}</math>
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<math>=(y+1)^p+(y+1)^{p-1}y+\cdots +(y+1)y^{p-1} +y^p</math>.
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For example:
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<math>(y+1)^2-y^2=(y+1)+y</math>
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<math>(y+1)^3-y^3=(y+1)^2+(y+1)y+y^2</math>
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<math>(y+1)^4-y^4=(y+1)^3+(y+1)^2y+(y+1)y^2+y^3</math>
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If we also know that <math>y\geq 0</math> then:
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<math>2y\leq (y+1)^2-y^2\leq 2(y+1)</math>
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<math>3y^2\leq (y+1)^3-y^3\leq 3(y+1)^2</math>
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<math>4y^3\leq (y+1)^4-y^4\leq 4(y+1)^3</math>
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<math>(p+1)y^p\leq (y+1)^{p+1}-y^{p+1}\leq (p+1)(y+1)^p</math>
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==Sum of Cubes==
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<math>1^3=1^2</math>
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<math>1^3+2^3 =3^2</math>
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<math>1^3 +2^3+3^3=6^2</math>
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<math>1^3+2^3+3^3+4^3=10^2</math>
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==Factorizations of Sums of Powers==
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<math>x^2-y^2=(x-y)(x+y)</math>
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<math>x^3-y^3=(x-y)(x^2+xy+y^2)</math>
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 +
<math>x^4-y^4=(x-y)(x^3+x^2y+xy^2+y^3)</math>
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Note that all these sums of powers can be factorized as follows:
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If we have a difference of powers of degree <math>n</math>, then
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<cmath>x^n-y^n=(x-y)(x^n+x^ay+x^by^2...+y^n)</cmath>
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where <math>a, b, c,...</math> are equivalent to <math>n-1, n-2, n-3,...</math> respectively.
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Note, of course, that some of the signs simply change when we have sum of powers instead of difference. Regardless, observe that the "longer" polynomial in the factorization is simply a binomial theorem expansion of the binomial <math>(x+y)^n</math>, except for the fact that the coefficient on each of the terms is <math>1</math>. This can be quite useful in problems that might have a sum of powers expression as well as an application of the binomial theorem.
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- icecreamrolls8
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==See Also==
 
* [[Factoring]]
 
* [[Factoring]]
* [[Difference of Squares]]
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* [[Difference of squares]], an extremely common specific case of this.
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* [[Binomial Theorem]]
  
 
[[Category:Algebra]]
 
[[Category:Algebra]]

Revision as of 13:43, 14 July 2021

The sum and difference of powers are powerful factoring techniques that, respectively, factor a sum or a difference of certain powers.

Sums of Odd Powers

$a^{2n+1}+b^{2n+1}=(a+b)(a^{2n}-a^{2n-1}b+a^{2n-2}b^2-\ldots-ab^{2n-1}+b^{2n})$

Differences of Powers

If $p$ is a positive integer and $x$ and $y$ are real numbers,

$x^{p+1}-y^{p+1}=(x-y)(x^p+x^{p-1}y+\cdots +xy^{p-1}+y^p)$

For example:

$x^2-y^2=(x-y)(x+y)$

$x^3-y^3=(x-y)(x^2+xy+y^2)$

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

Note that the number of terms in the long factor is equal to the exponent in the expression being factored.

An amazing thing happens when $x$ and $y$ differ by $1$, say, $x = y+1$. Then $x-y = 1$ and

$x^{p+1}-y^{p+1}=(y+1)^{p+1}-y^{p+1}$

$=(y+1)^p+(y+1)^{p-1}y+\cdots +(y+1)y^{p-1} +y^p$.

For example:

$(y+1)^2-y^2=(y+1)+y$

$(y+1)^3-y^3=(y+1)^2+(y+1)y+y^2$

$(y+1)^4-y^4=(y+1)^3+(y+1)^2y+(y+1)y^2+y^3$

If we also know that $y\geq 0$ then:

$2y\leq (y+1)^2-y^2\leq 2(y+1)$

$3y^2\leq (y+1)^3-y^3\leq 3(y+1)^2$

$4y^3\leq (y+1)^4-y^4\leq 4(y+1)^3$

$(p+1)y^p\leq (y+1)^{p+1}-y^{p+1}\leq (p+1)(y+1)^p$

Sum of Cubes

$1^3=1^2$

$1^3+2^3 =3^2$

$1^3 +2^3+3^3=6^2$

$1^3+2^3+3^3+4^3=10^2$

Factorizations of Sums of Powers

$x^2-y^2=(x-y)(x+y)$

$x^3-y^3=(x-y)(x^2+xy+y^2)$

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

Note that all these sums of powers can be factorized as follows:

If we have a difference of powers of degree $n$, then

\[x^n-y^n=(x-y)(x^n+x^ay+x^by^2...+y^n)\]

where $a, b, c,...$ are equivalent to $n-1, n-2, n-3,...$ respectively.

Note, of course, that some of the signs simply change when we have sum of powers instead of difference. Regardless, observe that the "longer" polynomial in the factorization is simply a binomial theorem expansion of the binomial $(x+y)^n$, except for the fact that the coefficient on each of the terms is $1$. This can be quite useful in problems that might have a sum of powers expression as well as an application of the binomial theorem.

- icecreamrolls8

See Also