Is it possible to prove $a+b'=b+a', c+d'=d+c' implies ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'$ without using...
$begingroup$
Suppose that we are restricted to addition $(+)$ and multiplication $(cdot)$, and are NOT allowed to use subtraction $(-)$ and division $(div)$. Moreover, addition $(+)$ and multiplication $(cdot)$ are defined only on $Bbb N$. For convenience, I write $ab$ instead of $acdot b$.
The reason that I am not allowed to appeal to subtraction $(-)$ is that I am defining multiplication over $Bbb Z$ from ONLY multiplication and addition defined on $Bbb N$. We are allowed to use all properties about addition and multiplication defined on $Bbb N$.
I would like to prove below theorem:
begin{equation}
left.begin{aligned}
a,b,c,d,a',b',c',d' in Bbb N\
a+b'=b+a'\
c+d'=d+c'
end{aligned}right} implies ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'
end{equation}
While it is easy to come up with the desired equation by using subtraction $(-)$ defined on $Bbb Z$, I have struggled but to no avail in not using subtraction $(-)$ defined on $Bbb Z$.
Please shed me some light. Thank you for your help!
I added my attempt in case subtraction defined on $Bbb Z$ is allowed:
$a+b'=b+a' iff a-b=a'-b'$ and $c+d'=d+c' iff c-d = c'-d'$. Multiply two equalities, we get $(a-b)(c-d) = (a'-b')(c'-d')$ or equivalently $ac-ad-bc+bd = a'c'-a'd'-b'c'+b'd'$ or equivalently $ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'$.
algebra-precalculus
$endgroup$
|
show 8 more comments
$begingroup$
Suppose that we are restricted to addition $(+)$ and multiplication $(cdot)$, and are NOT allowed to use subtraction $(-)$ and division $(div)$. Moreover, addition $(+)$ and multiplication $(cdot)$ are defined only on $Bbb N$. For convenience, I write $ab$ instead of $acdot b$.
The reason that I am not allowed to appeal to subtraction $(-)$ is that I am defining multiplication over $Bbb Z$ from ONLY multiplication and addition defined on $Bbb N$. We are allowed to use all properties about addition and multiplication defined on $Bbb N$.
I would like to prove below theorem:
begin{equation}
left.begin{aligned}
a,b,c,d,a',b',c',d' in Bbb N\
a+b'=b+a'\
c+d'=d+c'
end{aligned}right} implies ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'
end{equation}
While it is easy to come up with the desired equation by using subtraction $(-)$ defined on $Bbb Z$, I have struggled but to no avail in not using subtraction $(-)$ defined on $Bbb Z$.
Please shed me some light. Thank you for your help!
I added my attempt in case subtraction defined on $Bbb Z$ is allowed:
$a+b'=b+a' iff a-b=a'-b'$ and $c+d'=d+c' iff c-d = c'-d'$. Multiply two equalities, we get $(a-b)(c-d) = (a'-b')(c'-d')$ or equivalently $ac-ad-bc+bd = a'c'-a'd'-b'c'+b'd'$ or equivalently $ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'$.
algebra-precalculus
$endgroup$
2
$begingroup$
Well do you have the cancellation property?
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:11
$begingroup$
Do you have any reasonable argument for not using -?
$endgroup$
– Martín Vacas Vignolo
Dec 31 '18 at 10:12
$begingroup$
@KennyLau, We are allowed to use all properties about addition and multiplication.
$endgroup$
– Akira
Dec 31 '18 at 10:13
3
$begingroup$
Then you can convert your proof using subtraction to a proof using cancellation.
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:13
$begingroup$
@MartínVacasVignolo, I am defining multiplication over $Bbb Z$ from multiplication and addition over $Bbb N$.
$endgroup$
– Akira
Dec 31 '18 at 10:15
|
show 8 more comments
$begingroup$
Suppose that we are restricted to addition $(+)$ and multiplication $(cdot)$, and are NOT allowed to use subtraction $(-)$ and division $(div)$. Moreover, addition $(+)$ and multiplication $(cdot)$ are defined only on $Bbb N$. For convenience, I write $ab$ instead of $acdot b$.
The reason that I am not allowed to appeal to subtraction $(-)$ is that I am defining multiplication over $Bbb Z$ from ONLY multiplication and addition defined on $Bbb N$. We are allowed to use all properties about addition and multiplication defined on $Bbb N$.
I would like to prove below theorem:
begin{equation}
left.begin{aligned}
a,b,c,d,a',b',c',d' in Bbb N\
a+b'=b+a'\
c+d'=d+c'
end{aligned}right} implies ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'
end{equation}
While it is easy to come up with the desired equation by using subtraction $(-)$ defined on $Bbb Z$, I have struggled but to no avail in not using subtraction $(-)$ defined on $Bbb Z$.
Please shed me some light. Thank you for your help!
I added my attempt in case subtraction defined on $Bbb Z$ is allowed:
$a+b'=b+a' iff a-b=a'-b'$ and $c+d'=d+c' iff c-d = c'-d'$. Multiply two equalities, we get $(a-b)(c-d) = (a'-b')(c'-d')$ or equivalently $ac-ad-bc+bd = a'c'-a'd'-b'c'+b'd'$ or equivalently $ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'$.
algebra-precalculus
$endgroup$
Suppose that we are restricted to addition $(+)$ and multiplication $(cdot)$, and are NOT allowed to use subtraction $(-)$ and division $(div)$. Moreover, addition $(+)$ and multiplication $(cdot)$ are defined only on $Bbb N$. For convenience, I write $ab$ instead of $acdot b$.
The reason that I am not allowed to appeal to subtraction $(-)$ is that I am defining multiplication over $Bbb Z$ from ONLY multiplication and addition defined on $Bbb N$. We are allowed to use all properties about addition and multiplication defined on $Bbb N$.
I would like to prove below theorem:
begin{equation}
left.begin{aligned}
a,b,c,d,a',b',c',d' in Bbb N\
a+b'=b+a'\
c+d'=d+c'
end{aligned}right} implies ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'
end{equation}
While it is easy to come up with the desired equation by using subtraction $(-)$ defined on $Bbb Z$, I have struggled but to no avail in not using subtraction $(-)$ defined on $Bbb Z$.
Please shed me some light. Thank you for your help!
I added my attempt in case subtraction defined on $Bbb Z$ is allowed:
$a+b'=b+a' iff a-b=a'-b'$ and $c+d'=d+c' iff c-d = c'-d'$. Multiply two equalities, we get $(a-b)(c-d) = (a'-b')(c'-d')$ or equivalently $ac-ad-bc+bd = a'c'-a'd'-b'c'+b'd'$ or equivalently $ac+bd+a'd'+b'c' = ad+bc+a'c'+b'd'$.
algebra-precalculus
algebra-precalculus
edited Dec 31 '18 at 11:45
Akira
asked Dec 31 '18 at 10:08
AkiraAkira
21415
21415
2
$begingroup$
Well do you have the cancellation property?
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:11
$begingroup$
Do you have any reasonable argument for not using -?
$endgroup$
– Martín Vacas Vignolo
Dec 31 '18 at 10:12
$begingroup$
@KennyLau, We are allowed to use all properties about addition and multiplication.
$endgroup$
– Akira
Dec 31 '18 at 10:13
3
$begingroup$
Then you can convert your proof using subtraction to a proof using cancellation.
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:13
$begingroup$
@MartínVacasVignolo, I am defining multiplication over $Bbb Z$ from multiplication and addition over $Bbb N$.
$endgroup$
– Akira
Dec 31 '18 at 10:15
|
show 8 more comments
2
$begingroup$
Well do you have the cancellation property?
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:11
$begingroup$
Do you have any reasonable argument for not using -?
$endgroup$
– Martín Vacas Vignolo
Dec 31 '18 at 10:12
$begingroup$
@KennyLau, We are allowed to use all properties about addition and multiplication.
$endgroup$
– Akira
Dec 31 '18 at 10:13
3
$begingroup$
Then you can convert your proof using subtraction to a proof using cancellation.
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:13
$begingroup$
@MartínVacasVignolo, I am defining multiplication over $Bbb Z$ from multiplication and addition over $Bbb N$.
$endgroup$
– Akira
Dec 31 '18 at 10:15
2
2
$begingroup$
Well do you have the cancellation property?
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:11
$begingroup$
Well do you have the cancellation property?
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:11
$begingroup$
Do you have any reasonable argument for not using -?
$endgroup$
– Martín Vacas Vignolo
Dec 31 '18 at 10:12
$begingroup$
Do you have any reasonable argument for not using -?
$endgroup$
– Martín Vacas Vignolo
Dec 31 '18 at 10:12
$begingroup$
@KennyLau, We are allowed to use all properties about addition and multiplication.
$endgroup$
– Akira
Dec 31 '18 at 10:13
$begingroup$
@KennyLau, We are allowed to use all properties about addition and multiplication.
$endgroup$
– Akira
Dec 31 '18 at 10:13
3
3
$begingroup$
Then you can convert your proof using subtraction to a proof using cancellation.
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:13
$begingroup$
Then you can convert your proof using subtraction to a proof using cancellation.
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:13
$begingroup$
@MartínVacasVignolo, I am defining multiplication over $Bbb Z$ from multiplication and addition over $Bbb N$.
$endgroup$
– Akira
Dec 31 '18 at 10:15
$begingroup$
@MartínVacasVignolo, I am defining multiplication over $Bbb Z$ from multiplication and addition over $Bbb N$.
$endgroup$
– Akira
Dec 31 '18 at 10:15
|
show 8 more comments
1 Answer
1
active
oldest
votes
$begingroup$
I have figured out the proof and posted it here:
WLOG, we assume $a ge b$ and $c ge d$. Then $b' le a'$ and $d' le c'$.
It follows that there exist unique $overline b, overline {b'}, overline d, overline {d'}$ such that $a = b+overline b, a' = b' + overline {b'}, c = d + overline d, c' = d' + overline {d'}$.
Then $a+b'=b+a' iff (b+overline b) + b' = b + (b' + overline {b'}) iff overline b = overline {b'}$ and $c+d'=d+c' iff (d + overline d) + d' = d + (d' + overline {d'}) iff overline d = overline {d'}$.
Then $a = b+overline b, a' = b' + overline b, c = d + overline d, c' = d' + overline d$.
$LHS=ac+bd+a'd'+b'c' = (b+overline b) (d + overline d) +bd +(b' + overline b)d' + b'(d' + overline d) =bd+boverline d +overline b d+overline b overline d+bd+b'd'+overline bd'+b'd'+b'overline d=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
$RHS=ad+bc+a'c'+b'd'=(b+overline b)d+b(d + overline d)+(b' + overline b)(d' + overline d)+b'd'=bd+overline b d+bd+boverline d +b'd'+b'overline d+overline b d'+overline boverline d+b'd'=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
Thus $LHS=RHS$.
$endgroup$
add a comment |
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$begingroup$
I have figured out the proof and posted it here:
WLOG, we assume $a ge b$ and $c ge d$. Then $b' le a'$ and $d' le c'$.
It follows that there exist unique $overline b, overline {b'}, overline d, overline {d'}$ such that $a = b+overline b, a' = b' + overline {b'}, c = d + overline d, c' = d' + overline {d'}$.
Then $a+b'=b+a' iff (b+overline b) + b' = b + (b' + overline {b'}) iff overline b = overline {b'}$ and $c+d'=d+c' iff (d + overline d) + d' = d + (d' + overline {d'}) iff overline d = overline {d'}$.
Then $a = b+overline b, a' = b' + overline b, c = d + overline d, c' = d' + overline d$.
$LHS=ac+bd+a'd'+b'c' = (b+overline b) (d + overline d) +bd +(b' + overline b)d' + b'(d' + overline d) =bd+boverline d +overline b d+overline b overline d+bd+b'd'+overline bd'+b'd'+b'overline d=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
$RHS=ad+bc+a'c'+b'd'=(b+overline b)d+b(d + overline d)+(b' + overline b)(d' + overline d)+b'd'=bd+overline b d+bd+boverline d +b'd'+b'overline d+overline b d'+overline boverline d+b'd'=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
Thus $LHS=RHS$.
$endgroup$
add a comment |
$begingroup$
I have figured out the proof and posted it here:
WLOG, we assume $a ge b$ and $c ge d$. Then $b' le a'$ and $d' le c'$.
It follows that there exist unique $overline b, overline {b'}, overline d, overline {d'}$ such that $a = b+overline b, a' = b' + overline {b'}, c = d + overline d, c' = d' + overline {d'}$.
Then $a+b'=b+a' iff (b+overline b) + b' = b + (b' + overline {b'}) iff overline b = overline {b'}$ and $c+d'=d+c' iff (d + overline d) + d' = d + (d' + overline {d'}) iff overline d = overline {d'}$.
Then $a = b+overline b, a' = b' + overline b, c = d + overline d, c' = d' + overline d$.
$LHS=ac+bd+a'd'+b'c' = (b+overline b) (d + overline d) +bd +(b' + overline b)d' + b'(d' + overline d) =bd+boverline d +overline b d+overline b overline d+bd+b'd'+overline bd'+b'd'+b'overline d=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
$RHS=ad+bc+a'c'+b'd'=(b+overline b)d+b(d + overline d)+(b' + overline b)(d' + overline d)+b'd'=bd+overline b d+bd+boverline d +b'd'+b'overline d+overline b d'+overline boverline d+b'd'=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
Thus $LHS=RHS$.
$endgroup$
add a comment |
$begingroup$
I have figured out the proof and posted it here:
WLOG, we assume $a ge b$ and $c ge d$. Then $b' le a'$ and $d' le c'$.
It follows that there exist unique $overline b, overline {b'}, overline d, overline {d'}$ such that $a = b+overline b, a' = b' + overline {b'}, c = d + overline d, c' = d' + overline {d'}$.
Then $a+b'=b+a' iff (b+overline b) + b' = b + (b' + overline {b'}) iff overline b = overline {b'}$ and $c+d'=d+c' iff (d + overline d) + d' = d + (d' + overline {d'}) iff overline d = overline {d'}$.
Then $a = b+overline b, a' = b' + overline b, c = d + overline d, c' = d' + overline d$.
$LHS=ac+bd+a'd'+b'c' = (b+overline b) (d + overline d) +bd +(b' + overline b)d' + b'(d' + overline d) =bd+boverline d +overline b d+overline b overline d+bd+b'd'+overline bd'+b'd'+b'overline d=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
$RHS=ad+bc+a'c'+b'd'=(b+overline b)d+b(d + overline d)+(b' + overline b)(d' + overline d)+b'd'=bd+overline b d+bd+boverline d +b'd'+b'overline d+overline b d'+overline boverline d+b'd'=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
Thus $LHS=RHS$.
$endgroup$
I have figured out the proof and posted it here:
WLOG, we assume $a ge b$ and $c ge d$. Then $b' le a'$ and $d' le c'$.
It follows that there exist unique $overline b, overline {b'}, overline d, overline {d'}$ such that $a = b+overline b, a' = b' + overline {b'}, c = d + overline d, c' = d' + overline {d'}$.
Then $a+b'=b+a' iff (b+overline b) + b' = b + (b' + overline {b'}) iff overline b = overline {b'}$ and $c+d'=d+c' iff (d + overline d) + d' = d + (d' + overline {d'}) iff overline d = overline {d'}$.
Then $a = b+overline b, a' = b' + overline b, c = d + overline d, c' = d' + overline d$.
$LHS=ac+bd+a'd'+b'c' = (b+overline b) (d + overline d) +bd +(b' + overline b)d' + b'(d' + overline d) =bd+boverline d +overline b d+overline b overline d+bd+b'd'+overline bd'+b'd'+b'overline d=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
$RHS=ad+bc+a'c'+b'd'=(b+overline b)d+b(d + overline d)+(b' + overline b)(d' + overline d)+b'd'=bd+overline b d+bd+boverline d +b'd'+b'overline d+overline b d'+overline boverline d+b'd'=2(bd+b'd')+boverline d +overline b d+overline b overline d+overline bd'+b'overline d.$
Thus $LHS=RHS$.
answered Jan 1 at 2:58
AkiraAkira
21415
21415
add a comment |
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2
$begingroup$
Well do you have the cancellation property?
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:11
$begingroup$
Do you have any reasonable argument for not using -?
$endgroup$
– Martín Vacas Vignolo
Dec 31 '18 at 10:12
$begingroup$
@KennyLau, We are allowed to use all properties about addition and multiplication.
$endgroup$
– Akira
Dec 31 '18 at 10:13
3
$begingroup$
Then you can convert your proof using subtraction to a proof using cancellation.
$endgroup$
– Kenny Lau
Dec 31 '18 at 10:13
$begingroup$
@MartínVacasVignolo, I am defining multiplication over $Bbb Z$ from multiplication and addition over $Bbb N$.
$endgroup$
– Akira
Dec 31 '18 at 10:15