AMC 1834 - If $a, b $ and $c$ are nonzero numbers such that (a+b-c)/c=(a-b+c)/b=(-a+b+c)/a and...
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I am getting stuck on this question:
If a, b, and c, are nonzero numbers such that $frac{a+b-c}{c}$=$frac{a-b+c}{b}$=$frac{-a+b+c}{a}$ and x=$frac{(a+b)(b+c)(c+a)}{abc}$ and x<0, then x=?
What I have done so far: Consider, if $frac{a}{b}$=$frac{c}{d}$, and (b+d) is non-zero, then $frac{a+c}{b+d}$=$frac{a}{b}$=$frac{c}{d}$. So with that fact, I did... $frac{a+b-c}{c}$=$frac{a-b+c}{b}$ $implies$
$frac{2a}{c+b}$ and $frac{2a}{c+b}$=$frac{-a+b+c}{a}$$implies$ $frac{(a+b+c)}{a+b+c}$ = 1. Then I took the original equations and did $frac{a+b-c}{c}$=1, so $a+b=2c$ , and similarly, $(a+c)=2b$ and $b+c=2a$. So plugging that into x=$frac{(a+b)(b+c)(c+a)}{abc}$ we have $x=frac{(2a)(2b)(2c)}{abc}$, which is $x=frac{8(abc)}{abc}=8$. This is wrong though, because it says in the problem x<0. So how do I do this?
algebra-precalculus fractions
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add a comment |
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I am getting stuck on this question:
If a, b, and c, are nonzero numbers such that $frac{a+b-c}{c}$=$frac{a-b+c}{b}$=$frac{-a+b+c}{a}$ and x=$frac{(a+b)(b+c)(c+a)}{abc}$ and x<0, then x=?
What I have done so far: Consider, if $frac{a}{b}$=$frac{c}{d}$, and (b+d) is non-zero, then $frac{a+c}{b+d}$=$frac{a}{b}$=$frac{c}{d}$. So with that fact, I did... $frac{a+b-c}{c}$=$frac{a-b+c}{b}$ $implies$
$frac{2a}{c+b}$ and $frac{2a}{c+b}$=$frac{-a+b+c}{a}$$implies$ $frac{(a+b+c)}{a+b+c}$ = 1. Then I took the original equations and did $frac{a+b-c}{c}$=1, so $a+b=2c$ , and similarly, $(a+c)=2b$ and $b+c=2a$. So plugging that into x=$frac{(a+b)(b+c)(c+a)}{abc}$ we have $x=frac{(2a)(2b)(2c)}{abc}$, which is $x=frac{8(abc)}{abc}=8$. This is wrong though, because it says in the problem x<0. So how do I do this?
algebra-precalculus fractions
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add a comment |
$begingroup$
I am getting stuck on this question:
If a, b, and c, are nonzero numbers such that $frac{a+b-c}{c}$=$frac{a-b+c}{b}$=$frac{-a+b+c}{a}$ and x=$frac{(a+b)(b+c)(c+a)}{abc}$ and x<0, then x=?
What I have done so far: Consider, if $frac{a}{b}$=$frac{c}{d}$, and (b+d) is non-zero, then $frac{a+c}{b+d}$=$frac{a}{b}$=$frac{c}{d}$. So with that fact, I did... $frac{a+b-c}{c}$=$frac{a-b+c}{b}$ $implies$
$frac{2a}{c+b}$ and $frac{2a}{c+b}$=$frac{-a+b+c}{a}$$implies$ $frac{(a+b+c)}{a+b+c}$ = 1. Then I took the original equations and did $frac{a+b-c}{c}$=1, so $a+b=2c$ , and similarly, $(a+c)=2b$ and $b+c=2a$. So plugging that into x=$frac{(a+b)(b+c)(c+a)}{abc}$ we have $x=frac{(2a)(2b)(2c)}{abc}$, which is $x=frac{8(abc)}{abc}=8$. This is wrong though, because it says in the problem x<0. So how do I do this?
algebra-precalculus fractions
$endgroup$
I am getting stuck on this question:
If a, b, and c, are nonzero numbers such that $frac{a+b-c}{c}$=$frac{a-b+c}{b}$=$frac{-a+b+c}{a}$ and x=$frac{(a+b)(b+c)(c+a)}{abc}$ and x<0, then x=?
What I have done so far: Consider, if $frac{a}{b}$=$frac{c}{d}$, and (b+d) is non-zero, then $frac{a+c}{b+d}$=$frac{a}{b}$=$frac{c}{d}$. So with that fact, I did... $frac{a+b-c}{c}$=$frac{a-b+c}{b}$ $implies$
$frac{2a}{c+b}$ and $frac{2a}{c+b}$=$frac{-a+b+c}{a}$$implies$ $frac{(a+b+c)}{a+b+c}$ = 1. Then I took the original equations and did $frac{a+b-c}{c}$=1, so $a+b=2c$ , and similarly, $(a+c)=2b$ and $b+c=2a$. So plugging that into x=$frac{(a+b)(b+c)(c+a)}{abc}$ we have $x=frac{(2a)(2b)(2c)}{abc}$, which is $x=frac{8(abc)}{abc}=8$. This is wrong though, because it says in the problem x<0. So how do I do this?
algebra-precalculus fractions
algebra-precalculus fractions
edited Dec 20 '18 at 17:49
Ankit Kumar
1,535221
1,535221
asked Dec 20 '18 at 17:26
Kenneth DangKenneth Dang
136
136
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3 Answers
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$$frac{a+b-c}{c}=frac{a-b+c}{b}$$
$$ab+b^2-bc=ac-bc+c^2implies a(b-c)=(b+c)(c-b)$$
$$implies b=c quad ORquad a+b+c=0$$
If we take $b=c$, $$frac{a}{c}=frac{-a+2c}{a}quad using (1)and (3).$$
$$implies ({frac{a}{c}})^2=1quad ORquad -2$$
Obviosuly $-2$ isn't possible, $a=c$ will give $x=1$ and $a=-c$ will give $x=0$.
So, $a+b+c=0implies a=-(b+c), b=-(a+c), c=-(a+b)implies x=-1.$
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1
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Dear Mr. Kumar, how do you know from a(b-c)=(b+c)(c-b) that b=c or a+b+c=0?
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– Kenneth Dang
Dec 20 '18 at 18:44
2
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$a(b-c)=(b+c)(c-b)$ is same as $a(b-c)+(b-c)(b+c)=0$ which is same as $(b-c)(a+b+c)=0$. Btw thank you, no one addressed me like that before ;)
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– Ankit Kumar
Dec 20 '18 at 18:49
add a comment |
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Hint: $$a=-frac{b}{2},b=b,c=-frac{b}{2}$$ solves your problem and $$x=-1$$
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add a comment |
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You correctly derived:
$$frac{2a}{c+b}=frac{-a+b+c}{a}$$
Your mistake is here:
Then I took the original equations and did $frac{a+b-c}{c}=1$
Instead, denote: $frac{b+c}{a}=t$, then the above equation becomes:
$$frac2{t}=-1+t Rightarrow t^2-t-2=0 Rightarrow t_1=-1 text{and} t_2=2.$$
Also note that:
$$frac{a+b-c}{c}=frac{a-b+c}{b}=frac{-a+b+c}{a}iff \
frac{a+b}{c}-1=frac{a+c}{b}-1=frac{b+c}{a}-1 iff \
frac{a+b}{c}=frac{a+c}{b}=frac{b+c}{a}=t.$$
Hence:
$$x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=(-1)^3=-1;\
x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=2^3not< 0.$$
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Wow, I never thought of it like that. Do you an assertion as to when I should think similarly to this when dealing with problem solving?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 20:59
add a comment |
Your Answer
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3 Answers
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3 Answers
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$begingroup$
$$frac{a+b-c}{c}=frac{a-b+c}{b}$$
$$ab+b^2-bc=ac-bc+c^2implies a(b-c)=(b+c)(c-b)$$
$$implies b=c quad ORquad a+b+c=0$$
If we take $b=c$, $$frac{a}{c}=frac{-a+2c}{a}quad using (1)and (3).$$
$$implies ({frac{a}{c}})^2=1quad ORquad -2$$
Obviosuly $-2$ isn't possible, $a=c$ will give $x=1$ and $a=-c$ will give $x=0$.
So, $a+b+c=0implies a=-(b+c), b=-(a+c), c=-(a+b)implies x=-1.$
$endgroup$
1
$begingroup$
Dear Mr. Kumar, how do you know from a(b-c)=(b+c)(c-b) that b=c or a+b+c=0?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 18:44
2
$begingroup$
$a(b-c)=(b+c)(c-b)$ is same as $a(b-c)+(b-c)(b+c)=0$ which is same as $(b-c)(a+b+c)=0$. Btw thank you, no one addressed me like that before ;)
$endgroup$
– Ankit Kumar
Dec 20 '18 at 18:49
add a comment |
$begingroup$
$$frac{a+b-c}{c}=frac{a-b+c}{b}$$
$$ab+b^2-bc=ac-bc+c^2implies a(b-c)=(b+c)(c-b)$$
$$implies b=c quad ORquad a+b+c=0$$
If we take $b=c$, $$frac{a}{c}=frac{-a+2c}{a}quad using (1)and (3).$$
$$implies ({frac{a}{c}})^2=1quad ORquad -2$$
Obviosuly $-2$ isn't possible, $a=c$ will give $x=1$ and $a=-c$ will give $x=0$.
So, $a+b+c=0implies a=-(b+c), b=-(a+c), c=-(a+b)implies x=-1.$
$endgroup$
1
$begingroup$
Dear Mr. Kumar, how do you know from a(b-c)=(b+c)(c-b) that b=c or a+b+c=0?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 18:44
2
$begingroup$
$a(b-c)=(b+c)(c-b)$ is same as $a(b-c)+(b-c)(b+c)=0$ which is same as $(b-c)(a+b+c)=0$. Btw thank you, no one addressed me like that before ;)
$endgroup$
– Ankit Kumar
Dec 20 '18 at 18:49
add a comment |
$begingroup$
$$frac{a+b-c}{c}=frac{a-b+c}{b}$$
$$ab+b^2-bc=ac-bc+c^2implies a(b-c)=(b+c)(c-b)$$
$$implies b=c quad ORquad a+b+c=0$$
If we take $b=c$, $$frac{a}{c}=frac{-a+2c}{a}quad using (1)and (3).$$
$$implies ({frac{a}{c}})^2=1quad ORquad -2$$
Obviosuly $-2$ isn't possible, $a=c$ will give $x=1$ and $a=-c$ will give $x=0$.
So, $a+b+c=0implies a=-(b+c), b=-(a+c), c=-(a+b)implies x=-1.$
$endgroup$
$$frac{a+b-c}{c}=frac{a-b+c}{b}$$
$$ab+b^2-bc=ac-bc+c^2implies a(b-c)=(b+c)(c-b)$$
$$implies b=c quad ORquad a+b+c=0$$
If we take $b=c$, $$frac{a}{c}=frac{-a+2c}{a}quad using (1)and (3).$$
$$implies ({frac{a}{c}})^2=1quad ORquad -2$$
Obviosuly $-2$ isn't possible, $a=c$ will give $x=1$ and $a=-c$ will give $x=0$.
So, $a+b+c=0implies a=-(b+c), b=-(a+c), c=-(a+b)implies x=-1.$
answered Dec 20 '18 at 17:43
Ankit KumarAnkit Kumar
1,535221
1,535221
1
$begingroup$
Dear Mr. Kumar, how do you know from a(b-c)=(b+c)(c-b) that b=c or a+b+c=0?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 18:44
2
$begingroup$
$a(b-c)=(b+c)(c-b)$ is same as $a(b-c)+(b-c)(b+c)=0$ which is same as $(b-c)(a+b+c)=0$. Btw thank you, no one addressed me like that before ;)
$endgroup$
– Ankit Kumar
Dec 20 '18 at 18:49
add a comment |
1
$begingroup$
Dear Mr. Kumar, how do you know from a(b-c)=(b+c)(c-b) that b=c or a+b+c=0?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 18:44
2
$begingroup$
$a(b-c)=(b+c)(c-b)$ is same as $a(b-c)+(b-c)(b+c)=0$ which is same as $(b-c)(a+b+c)=0$. Btw thank you, no one addressed me like that before ;)
$endgroup$
– Ankit Kumar
Dec 20 '18 at 18:49
1
1
$begingroup$
Dear Mr. Kumar, how do you know from a(b-c)=(b+c)(c-b) that b=c or a+b+c=0?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 18:44
$begingroup$
Dear Mr. Kumar, how do you know from a(b-c)=(b+c)(c-b) that b=c or a+b+c=0?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 18:44
2
2
$begingroup$
$a(b-c)=(b+c)(c-b)$ is same as $a(b-c)+(b-c)(b+c)=0$ which is same as $(b-c)(a+b+c)=0$. Btw thank you, no one addressed me like that before ;)
$endgroup$
– Ankit Kumar
Dec 20 '18 at 18:49
$begingroup$
$a(b-c)=(b+c)(c-b)$ is same as $a(b-c)+(b-c)(b+c)=0$ which is same as $(b-c)(a+b+c)=0$. Btw thank you, no one addressed me like that before ;)
$endgroup$
– Ankit Kumar
Dec 20 '18 at 18:49
add a comment |
$begingroup$
Hint: $$a=-frac{b}{2},b=b,c=-frac{b}{2}$$ solves your problem and $$x=-1$$
$endgroup$
add a comment |
$begingroup$
Hint: $$a=-frac{b}{2},b=b,c=-frac{b}{2}$$ solves your problem and $$x=-1$$
$endgroup$
add a comment |
$begingroup$
Hint: $$a=-frac{b}{2},b=b,c=-frac{b}{2}$$ solves your problem and $$x=-1$$
$endgroup$
Hint: $$a=-frac{b}{2},b=b,c=-frac{b}{2}$$ solves your problem and $$x=-1$$
answered Dec 20 '18 at 17:44
Dr. Sonnhard GraubnerDr. Sonnhard Graubner
78k42866
78k42866
add a comment |
add a comment |
$begingroup$
You correctly derived:
$$frac{2a}{c+b}=frac{-a+b+c}{a}$$
Your mistake is here:
Then I took the original equations and did $frac{a+b-c}{c}=1$
Instead, denote: $frac{b+c}{a}=t$, then the above equation becomes:
$$frac2{t}=-1+t Rightarrow t^2-t-2=0 Rightarrow t_1=-1 text{and} t_2=2.$$
Also note that:
$$frac{a+b-c}{c}=frac{a-b+c}{b}=frac{-a+b+c}{a}iff \
frac{a+b}{c}-1=frac{a+c}{b}-1=frac{b+c}{a}-1 iff \
frac{a+b}{c}=frac{a+c}{b}=frac{b+c}{a}=t.$$
Hence:
$$x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=(-1)^3=-1;\
x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=2^3not< 0.$$
$endgroup$
$begingroup$
Wow, I never thought of it like that. Do you an assertion as to when I should think similarly to this when dealing with problem solving?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 20:59
add a comment |
$begingroup$
You correctly derived:
$$frac{2a}{c+b}=frac{-a+b+c}{a}$$
Your mistake is here:
Then I took the original equations and did $frac{a+b-c}{c}=1$
Instead, denote: $frac{b+c}{a}=t$, then the above equation becomes:
$$frac2{t}=-1+t Rightarrow t^2-t-2=0 Rightarrow t_1=-1 text{and} t_2=2.$$
Also note that:
$$frac{a+b-c}{c}=frac{a-b+c}{b}=frac{-a+b+c}{a}iff \
frac{a+b}{c}-1=frac{a+c}{b}-1=frac{b+c}{a}-1 iff \
frac{a+b}{c}=frac{a+c}{b}=frac{b+c}{a}=t.$$
Hence:
$$x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=(-1)^3=-1;\
x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=2^3not< 0.$$
$endgroup$
$begingroup$
Wow, I never thought of it like that. Do you an assertion as to when I should think similarly to this when dealing with problem solving?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 20:59
add a comment |
$begingroup$
You correctly derived:
$$frac{2a}{c+b}=frac{-a+b+c}{a}$$
Your mistake is here:
Then I took the original equations and did $frac{a+b-c}{c}=1$
Instead, denote: $frac{b+c}{a}=t$, then the above equation becomes:
$$frac2{t}=-1+t Rightarrow t^2-t-2=0 Rightarrow t_1=-1 text{and} t_2=2.$$
Also note that:
$$frac{a+b-c}{c}=frac{a-b+c}{b}=frac{-a+b+c}{a}iff \
frac{a+b}{c}-1=frac{a+c}{b}-1=frac{b+c}{a}-1 iff \
frac{a+b}{c}=frac{a+c}{b}=frac{b+c}{a}=t.$$
Hence:
$$x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=(-1)^3=-1;\
x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=2^3not< 0.$$
$endgroup$
You correctly derived:
$$frac{2a}{c+b}=frac{-a+b+c}{a}$$
Your mistake is here:
Then I took the original equations and did $frac{a+b-c}{c}=1$
Instead, denote: $frac{b+c}{a}=t$, then the above equation becomes:
$$frac2{t}=-1+t Rightarrow t^2-t-2=0 Rightarrow t_1=-1 text{and} t_2=2.$$
Also note that:
$$frac{a+b-c}{c}=frac{a-b+c}{b}=frac{-a+b+c}{a}iff \
frac{a+b}{c}-1=frac{a+c}{b}-1=frac{b+c}{a}-1 iff \
frac{a+b}{c}=frac{a+c}{b}=frac{b+c}{a}=t.$$
Hence:
$$x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=(-1)^3=-1;\
x=frac{(a+b)(b+c)(c+a)}{abc}=t^3=2^3not< 0.$$
answered Dec 20 '18 at 19:40
farruhotafarruhota
21.6k2842
21.6k2842
$begingroup$
Wow, I never thought of it like that. Do you an assertion as to when I should think similarly to this when dealing with problem solving?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 20:59
add a comment |
$begingroup$
Wow, I never thought of it like that. Do you an assertion as to when I should think similarly to this when dealing with problem solving?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 20:59
$begingroup$
Wow, I never thought of it like that. Do you an assertion as to when I should think similarly to this when dealing with problem solving?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 20:59
$begingroup$
Wow, I never thought of it like that. Do you an assertion as to when I should think similarly to this when dealing with problem solving?
$endgroup$
– Kenneth Dang
Dec 20 '18 at 20:59
add a comment |
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