The distributive law
$begingroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$ , but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
algebra-precalculus arithmetic
$endgroup$
add a comment |
$begingroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$ , but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
algebra-precalculus arithmetic
$endgroup$
$begingroup$
It is also the distributive law or rule.
$endgroup$
– Dr. Sonnhard Graubner
Dec 14 '18 at 16:19
$begingroup$
This is distribution done twice.
$endgroup$
– Randall
Dec 14 '18 at 16:20
add a comment |
$begingroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$ , but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
algebra-precalculus arithmetic
$endgroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$ , but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
algebra-precalculus arithmetic
algebra-precalculus arithmetic
asked Dec 14 '18 at 16:18
Ashraf BenmebarekAshraf Benmebarek
465
465
$begingroup$
It is also the distributive law or rule.
$endgroup$
– Dr. Sonnhard Graubner
Dec 14 '18 at 16:19
$begingroup$
This is distribution done twice.
$endgroup$
– Randall
Dec 14 '18 at 16:20
add a comment |
$begingroup$
It is also the distributive law or rule.
$endgroup$
– Dr. Sonnhard Graubner
Dec 14 '18 at 16:19
$begingroup$
This is distribution done twice.
$endgroup$
– Randall
Dec 14 '18 at 16:20
$begingroup$
It is also the distributive law or rule.
$endgroup$
– Dr. Sonnhard Graubner
Dec 14 '18 at 16:19
$begingroup$
It is also the distributive law or rule.
$endgroup$
– Dr. Sonnhard Graubner
Dec 14 '18 at 16:19
$begingroup$
This is distribution done twice.
$endgroup$
– Randall
Dec 14 '18 at 16:20
$begingroup$
This is distribution done twice.
$endgroup$
– Randall
Dec 14 '18 at 16:20
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Because:
$$left (x+y right) left (x+y right) = underbrace{(x+y)+(x+y)+...(x+y)}_{x + ytext{ times}}$$
$$=underbrace{(x+y)+(x+y)+...(x+y)}_{xtext{ times}}+underbrace{(x+y)+(x+y)+...(x+y)}_{ytext{ times}}$$
$$=underbrace{x}_{xtext{ times}}+underbrace{y}_{xtext{ times}}+underbrace{x}_{ytext{ times}}+underbrace{y}_{ytext{ times}}$$
$$=xx+xy+yx+yy$$
$endgroup$
$begingroup$
If $x+y$ isn't an integer, what does "$x+y$ times" mean then? The same for $x$ and $y$ separately.
$endgroup$
– StackTD
Dec 14 '18 at 16:30
$begingroup$
@StackTD Excellent question! Obviously one needs to do something else in that case ... but I figured this would be in the spirit of the OP
$endgroup$
– Bram28
Dec 14 '18 at 16:32
$begingroup$
I'm not sure what OP means, but my comment is intended to make OP think about that too because even in his first example, $x$ and $y$ need not be integers (and the argument still holds, thanks to the 4!).
$endgroup$
– StackTD
Dec 14 '18 at 16:33
add a comment |
$begingroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$
This works because $4$ is a (positive) integer; but for non-integer factors you can't use the "repeated addition"-argument.
but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
The distributive law works even if $x$ and/or $y$ are not integers.
Keep one of the factors together in a first step, and apply distributivity twice:
$$begin{align}
(color{blue}{x}+color{red}{y})(x+y) & =color{blue}{x}(x+y)+color{red}{y}(x+y) \ & =color{blue}{x}x+color{blue}{x}y+color{red}{y}x+color{red}{y}y
end{align}$$
$endgroup$
add a comment |
$begingroup$
Do it in steps.
You accept that $M(x+y) = Mx + My$
So replace $M$ with $(x+y)$ and you get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y$
Now distribute a second time: Replace $x$ with $A$ and $y$ with $B$ to get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y=$
$(x+y)A + (x+y)B=$
$xA + yA + xB + yB =$
$xx + yx + xy + yy =$
$x^2 + 2xy + y^2$.
Of you you don't have to, and you shouldn't, do all that replacement. You should do it directly.
$(x+y)(x+y)=$ we treat one of the $(x+y)$ as a single thing and distribute across the other $x + y$.
$(x+y)(x+y) = (x+y)x + (x+y)y=$.
Now we have to sums to distribute: $(x+y)x = xx + yx$ and $(x + y)y = xy + yy$. So puting them together:
$(x+y)(x+y) = (x+y)x + (x+y)y=xx + yx + xy + yy=$.
And then some clean up:
$=x^2 + 2xy + y^2$
$endgroup$
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3039567%2fthe-distributive-law%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Because:
$$left (x+y right) left (x+y right) = underbrace{(x+y)+(x+y)+...(x+y)}_{x + ytext{ times}}$$
$$=underbrace{(x+y)+(x+y)+...(x+y)}_{xtext{ times}}+underbrace{(x+y)+(x+y)+...(x+y)}_{ytext{ times}}$$
$$=underbrace{x}_{xtext{ times}}+underbrace{y}_{xtext{ times}}+underbrace{x}_{ytext{ times}}+underbrace{y}_{ytext{ times}}$$
$$=xx+xy+yx+yy$$
$endgroup$
$begingroup$
If $x+y$ isn't an integer, what does "$x+y$ times" mean then? The same for $x$ and $y$ separately.
$endgroup$
– StackTD
Dec 14 '18 at 16:30
$begingroup$
@StackTD Excellent question! Obviously one needs to do something else in that case ... but I figured this would be in the spirit of the OP
$endgroup$
– Bram28
Dec 14 '18 at 16:32
$begingroup$
I'm not sure what OP means, but my comment is intended to make OP think about that too because even in his first example, $x$ and $y$ need not be integers (and the argument still holds, thanks to the 4!).
$endgroup$
– StackTD
Dec 14 '18 at 16:33
add a comment |
$begingroup$
Because:
$$left (x+y right) left (x+y right) = underbrace{(x+y)+(x+y)+...(x+y)}_{x + ytext{ times}}$$
$$=underbrace{(x+y)+(x+y)+...(x+y)}_{xtext{ times}}+underbrace{(x+y)+(x+y)+...(x+y)}_{ytext{ times}}$$
$$=underbrace{x}_{xtext{ times}}+underbrace{y}_{xtext{ times}}+underbrace{x}_{ytext{ times}}+underbrace{y}_{ytext{ times}}$$
$$=xx+xy+yx+yy$$
$endgroup$
$begingroup$
If $x+y$ isn't an integer, what does "$x+y$ times" mean then? The same for $x$ and $y$ separately.
$endgroup$
– StackTD
Dec 14 '18 at 16:30
$begingroup$
@StackTD Excellent question! Obviously one needs to do something else in that case ... but I figured this would be in the spirit of the OP
$endgroup$
– Bram28
Dec 14 '18 at 16:32
$begingroup$
I'm not sure what OP means, but my comment is intended to make OP think about that too because even in his first example, $x$ and $y$ need not be integers (and the argument still holds, thanks to the 4!).
$endgroup$
– StackTD
Dec 14 '18 at 16:33
add a comment |
$begingroup$
Because:
$$left (x+y right) left (x+y right) = underbrace{(x+y)+(x+y)+...(x+y)}_{x + ytext{ times}}$$
$$=underbrace{(x+y)+(x+y)+...(x+y)}_{xtext{ times}}+underbrace{(x+y)+(x+y)+...(x+y)}_{ytext{ times}}$$
$$=underbrace{x}_{xtext{ times}}+underbrace{y}_{xtext{ times}}+underbrace{x}_{ytext{ times}}+underbrace{y}_{ytext{ times}}$$
$$=xx+xy+yx+yy$$
$endgroup$
Because:
$$left (x+y right) left (x+y right) = underbrace{(x+y)+(x+y)+...(x+y)}_{x + ytext{ times}}$$
$$=underbrace{(x+y)+(x+y)+...(x+y)}_{xtext{ times}}+underbrace{(x+y)+(x+y)+...(x+y)}_{ytext{ times}}$$
$$=underbrace{x}_{xtext{ times}}+underbrace{y}_{xtext{ times}}+underbrace{x}_{ytext{ times}}+underbrace{y}_{ytext{ times}}$$
$$=xx+xy+yx+yy$$
edited Dec 14 '18 at 16:30
answered Dec 14 '18 at 16:24
Bram28Bram28
63.2k44793
63.2k44793
$begingroup$
If $x+y$ isn't an integer, what does "$x+y$ times" mean then? The same for $x$ and $y$ separately.
$endgroup$
– StackTD
Dec 14 '18 at 16:30
$begingroup$
@StackTD Excellent question! Obviously one needs to do something else in that case ... but I figured this would be in the spirit of the OP
$endgroup$
– Bram28
Dec 14 '18 at 16:32
$begingroup$
I'm not sure what OP means, but my comment is intended to make OP think about that too because even in his first example, $x$ and $y$ need not be integers (and the argument still holds, thanks to the 4!).
$endgroup$
– StackTD
Dec 14 '18 at 16:33
add a comment |
$begingroup$
If $x+y$ isn't an integer, what does "$x+y$ times" mean then? The same for $x$ and $y$ separately.
$endgroup$
– StackTD
Dec 14 '18 at 16:30
$begingroup$
@StackTD Excellent question! Obviously one needs to do something else in that case ... but I figured this would be in the spirit of the OP
$endgroup$
– Bram28
Dec 14 '18 at 16:32
$begingroup$
I'm not sure what OP means, but my comment is intended to make OP think about that too because even in his first example, $x$ and $y$ need not be integers (and the argument still holds, thanks to the 4!).
$endgroup$
– StackTD
Dec 14 '18 at 16:33
$begingroup$
If $x+y$ isn't an integer, what does "$x+y$ times" mean then? The same for $x$ and $y$ separately.
$endgroup$
– StackTD
Dec 14 '18 at 16:30
$begingroup$
If $x+y$ isn't an integer, what does "$x+y$ times" mean then? The same for $x$ and $y$ separately.
$endgroup$
– StackTD
Dec 14 '18 at 16:30
$begingroup$
@StackTD Excellent question! Obviously one needs to do something else in that case ... but I figured this would be in the spirit of the OP
$endgroup$
– Bram28
Dec 14 '18 at 16:32
$begingroup$
@StackTD Excellent question! Obviously one needs to do something else in that case ... but I figured this would be in the spirit of the OP
$endgroup$
– Bram28
Dec 14 '18 at 16:32
$begingroup$
I'm not sure what OP means, but my comment is intended to make OP think about that too because even in his first example, $x$ and $y$ need not be integers (and the argument still holds, thanks to the 4!).
$endgroup$
– StackTD
Dec 14 '18 at 16:33
$begingroup$
I'm not sure what OP means, but my comment is intended to make OP think about that too because even in his first example, $x$ and $y$ need not be integers (and the argument still holds, thanks to the 4!).
$endgroup$
– StackTD
Dec 14 '18 at 16:33
add a comment |
$begingroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$
This works because $4$ is a (positive) integer; but for non-integer factors you can't use the "repeated addition"-argument.
but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
The distributive law works even if $x$ and/or $y$ are not integers.
Keep one of the factors together in a first step, and apply distributivity twice:
$$begin{align}
(color{blue}{x}+color{red}{y})(x+y) & =color{blue}{x}(x+y)+color{red}{y}(x+y) \ & =color{blue}{x}x+color{blue}{x}y+color{red}{y}x+color{red}{y}y
end{align}$$
$endgroup$
add a comment |
$begingroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$
This works because $4$ is a (positive) integer; but for non-integer factors you can't use the "repeated addition"-argument.
but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
The distributive law works even if $x$ and/or $y$ are not integers.
Keep one of the factors together in a first step, and apply distributivity twice:
$$begin{align}
(color{blue}{x}+color{red}{y})(x+y) & =color{blue}{x}(x+y)+color{red}{y}(x+y) \ & =color{blue}{x}x+color{blue}{x}y+color{red}{y}x+color{red}{y}y
end{align}$$
$endgroup$
add a comment |
$begingroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$
This works because $4$ is a (positive) integer; but for non-integer factors you can't use the "repeated addition"-argument.
but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
The distributive law works even if $x$ and/or $y$ are not integers.
Keep one of the factors together in a first step, and apply distributivity twice:
$$begin{align}
(color{blue}{x}+color{red}{y})(x+y) & =color{blue}{x}(x+y)+color{red}{y}(x+y) \ & =color{blue}{x}x+color{blue}{x}y+color{red}{y}x+color{red}{y}y
end{align}$$
$endgroup$
$4left (x+y right)=4x+4y $ because $4left (x+y right) =left (x+y right) +left (x+y right) +left (x+y right) +left (x+y right)$
This works because $4$ is a (positive) integer; but for non-integer factors you can't use the "repeated addition"-argument.
but why is $left (x+y right) left (x+y right) =xx+xy+yx+yy$?
The distributive law works even if $x$ and/or $y$ are not integers.
Keep one of the factors together in a first step, and apply distributivity twice:
$$begin{align}
(color{blue}{x}+color{red}{y})(x+y) & =color{blue}{x}(x+y)+color{red}{y}(x+y) \ & =color{blue}{x}x+color{blue}{x}y+color{red}{y}x+color{red}{y}y
end{align}$$
answered Dec 14 '18 at 16:20
StackTDStackTD
22.9k2152
22.9k2152
add a comment |
add a comment |
$begingroup$
Do it in steps.
You accept that $M(x+y) = Mx + My$
So replace $M$ with $(x+y)$ and you get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y$
Now distribute a second time: Replace $x$ with $A$ and $y$ with $B$ to get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y=$
$(x+y)A + (x+y)B=$
$xA + yA + xB + yB =$
$xx + yx + xy + yy =$
$x^2 + 2xy + y^2$.
Of you you don't have to, and you shouldn't, do all that replacement. You should do it directly.
$(x+y)(x+y)=$ we treat one of the $(x+y)$ as a single thing and distribute across the other $x + y$.
$(x+y)(x+y) = (x+y)x + (x+y)y=$.
Now we have to sums to distribute: $(x+y)x = xx + yx$ and $(x + y)y = xy + yy$. So puting them together:
$(x+y)(x+y) = (x+y)x + (x+y)y=xx + yx + xy + yy=$.
And then some clean up:
$=x^2 + 2xy + y^2$
$endgroup$
add a comment |
$begingroup$
Do it in steps.
You accept that $M(x+y) = Mx + My$
So replace $M$ with $(x+y)$ and you get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y$
Now distribute a second time: Replace $x$ with $A$ and $y$ with $B$ to get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y=$
$(x+y)A + (x+y)B=$
$xA + yA + xB + yB =$
$xx + yx + xy + yy =$
$x^2 + 2xy + y^2$.
Of you you don't have to, and you shouldn't, do all that replacement. You should do it directly.
$(x+y)(x+y)=$ we treat one of the $(x+y)$ as a single thing and distribute across the other $x + y$.
$(x+y)(x+y) = (x+y)x + (x+y)y=$.
Now we have to sums to distribute: $(x+y)x = xx + yx$ and $(x + y)y = xy + yy$. So puting them together:
$(x+y)(x+y) = (x+y)x + (x+y)y=xx + yx + xy + yy=$.
And then some clean up:
$=x^2 + 2xy + y^2$
$endgroup$
add a comment |
$begingroup$
Do it in steps.
You accept that $M(x+y) = Mx + My$
So replace $M$ with $(x+y)$ and you get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y$
Now distribute a second time: Replace $x$ with $A$ and $y$ with $B$ to get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y=$
$(x+y)A + (x+y)B=$
$xA + yA + xB + yB =$
$xx + yx + xy + yy =$
$x^2 + 2xy + y^2$.
Of you you don't have to, and you shouldn't, do all that replacement. You should do it directly.
$(x+y)(x+y)=$ we treat one of the $(x+y)$ as a single thing and distribute across the other $x + y$.
$(x+y)(x+y) = (x+y)x + (x+y)y=$.
Now we have to sums to distribute: $(x+y)x = xx + yx$ and $(x + y)y = xy + yy$. So puting them together:
$(x+y)(x+y) = (x+y)x + (x+y)y=xx + yx + xy + yy=$.
And then some clean up:
$=x^2 + 2xy + y^2$
$endgroup$
Do it in steps.
You accept that $M(x+y) = Mx + My$
So replace $M$ with $(x+y)$ and you get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y$
Now distribute a second time: Replace $x$ with $A$ and $y$ with $B$ to get:
$(x+y)(x+y) = M(x+y) =$
$Mx + My = $
$(x + y)x + (x+y)y=$
$(x+y)A + (x+y)B=$
$xA + yA + xB + yB =$
$xx + yx + xy + yy =$
$x^2 + 2xy + y^2$.
Of you you don't have to, and you shouldn't, do all that replacement. You should do it directly.
$(x+y)(x+y)=$ we treat one of the $(x+y)$ as a single thing and distribute across the other $x + y$.
$(x+y)(x+y) = (x+y)x + (x+y)y=$.
Now we have to sums to distribute: $(x+y)x = xx + yx$ and $(x + y)y = xy + yy$. So puting them together:
$(x+y)(x+y) = (x+y)x + (x+y)y=xx + yx + xy + yy=$.
And then some clean up:
$=x^2 + 2xy + y^2$
answered Dec 14 '18 at 16:51
fleabloodfleablood
71.7k22686
71.7k22686
add a comment |
add a comment |
Thanks for contributing an answer to Mathematics Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3039567%2fthe-distributive-law%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
$begingroup$
It is also the distributive law or rule.
$endgroup$
– Dr. Sonnhard Graubner
Dec 14 '18 at 16:19
$begingroup$
This is distribution done twice.
$endgroup$
– Randall
Dec 14 '18 at 16:20