Find the remainder of the division of $x^n+5$ with $x^3+10x^2+25x$
$begingroup$
Find the remainder of the division of $x^n+5$ with $x^3+10x^2+25x$ over $mathbb{Q}$
What I tried to do is to write $x^n+5=p(x)(x^3+10x^2+25x)+Ax^2+Bx+C$, where $p(x)$ is a polynomial of degree $n-3$. If I set $x=0$ I obtain that $C=5$.
Now $x^3+10x^2+25x=x(x+5)^2$ and by setting $x=-5$, I get that
$5A-B=(-1)^n5^{n-1}$
But I need one more equation to be able to find the coefficients of the remainder, and I can't get one. What should I do?
algebra-precalculus
$endgroup$
add a comment |
$begingroup$
Find the remainder of the division of $x^n+5$ with $x^3+10x^2+25x$ over $mathbb{Q}$
What I tried to do is to write $x^n+5=p(x)(x^3+10x^2+25x)+Ax^2+Bx+C$, where $p(x)$ is a polynomial of degree $n-3$. If I set $x=0$ I obtain that $C=5$.
Now $x^3+10x^2+25x=x(x+5)^2$ and by setting $x=-5$, I get that
$5A-B=(-1)^n5^{n-1}$
But I need one more equation to be able to find the coefficients of the remainder, and I can't get one. What should I do?
algebra-precalculus
$endgroup$
1
$begingroup$
Don't you get $C=5$ by setting $x=0$?
$endgroup$
– Alejandro Nasif Salum
Jan 29 at 19:54
1
$begingroup$
Hint: differentiate, since $,x = -5,$ is a double root you can again evaluate at $,x = -5,$ to get another equation in $A,B$
$endgroup$
– Bill Dubuque
Jan 29 at 19:58
add a comment |
$begingroup$
Find the remainder of the division of $x^n+5$ with $x^3+10x^2+25x$ over $mathbb{Q}$
What I tried to do is to write $x^n+5=p(x)(x^3+10x^2+25x)+Ax^2+Bx+C$, where $p(x)$ is a polynomial of degree $n-3$. If I set $x=0$ I obtain that $C=5$.
Now $x^3+10x^2+25x=x(x+5)^2$ and by setting $x=-5$, I get that
$5A-B=(-1)^n5^{n-1}$
But I need one more equation to be able to find the coefficients of the remainder, and I can't get one. What should I do?
algebra-precalculus
$endgroup$
Find the remainder of the division of $x^n+5$ with $x^3+10x^2+25x$ over $mathbb{Q}$
What I tried to do is to write $x^n+5=p(x)(x^3+10x^2+25x)+Ax^2+Bx+C$, where $p(x)$ is a polynomial of degree $n-3$. If I set $x=0$ I obtain that $C=5$.
Now $x^3+10x^2+25x=x(x+5)^2$ and by setting $x=-5$, I get that
$5A-B=(-1)^n5^{n-1}$
But I need one more equation to be able to find the coefficients of the remainder, and I can't get one. What should I do?
algebra-precalculus
algebra-precalculus
edited Jan 29 at 20:21
user3701033
asked Jan 29 at 19:51
user3701033user3701033
1519
1519
1
$begingroup$
Don't you get $C=5$ by setting $x=0$?
$endgroup$
– Alejandro Nasif Salum
Jan 29 at 19:54
1
$begingroup$
Hint: differentiate, since $,x = -5,$ is a double root you can again evaluate at $,x = -5,$ to get another equation in $A,B$
$endgroup$
– Bill Dubuque
Jan 29 at 19:58
add a comment |
1
$begingroup$
Don't you get $C=5$ by setting $x=0$?
$endgroup$
– Alejandro Nasif Salum
Jan 29 at 19:54
1
$begingroup$
Hint: differentiate, since $,x = -5,$ is a double root you can again evaluate at $,x = -5,$ to get another equation in $A,B$
$endgroup$
– Bill Dubuque
Jan 29 at 19:58
1
1
$begingroup$
Don't you get $C=5$ by setting $x=0$?
$endgroup$
– Alejandro Nasif Salum
Jan 29 at 19:54
$begingroup$
Don't you get $C=5$ by setting $x=0$?
$endgroup$
– Alejandro Nasif Salum
Jan 29 at 19:54
1
1
$begingroup$
Hint: differentiate, since $,x = -5,$ is a double root you can again evaluate at $,x = -5,$ to get another equation in $A,B$
$endgroup$
– Bill Dubuque
Jan 29 at 19:58
$begingroup$
Hint: differentiate, since $,x = -5,$ is a double root you can again evaluate at $,x = -5,$ to get another equation in $A,B$
$endgroup$
– Bill Dubuque
Jan 29 at 19:58
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
It is a good idea to use the roots of $q(x)=x^3+10x^2+25x$, but yes: because $-5$ is a double root, things get a little bit trickier. (Also, check that setting $x=0$ gives $C=5$, not $C=0$).
HINT: But since $-5$ is a double root of $q$, it is both a root of $q$ and $q'$. So try to take the derivative at both members and then take again $x=-5$.
$endgroup$
add a comment |
$begingroup$
Hint $ $ Subtracting $, C = 5,$ then cancelling $x$ yields
$qquad f(x) := x^{large n-1} = B + Ax + (x+5)^{large 2} p(x) $
Thus $, B + Ax = f(-5) + f'(-5)(x+5),$ by Taylor expansion at $,x = -5,$
$endgroup$
$begingroup$
If you don't know (or can't use) Taylor then you could instead apply this (purely algebraic) Polynomial Double Root Test. on $,x^{n-1}!-B-Ax,,$ or just differentiate as I hinted on the question.
$endgroup$
– Bill Dubuque
Jan 29 at 20:22
add a comment |
$begingroup$
I think there is an error in the attempt that is part of the question. Anyway, here are some relevant quotients and remainders:
$$ left( x^{4} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x - 10 right) + left( 75 x^{2} + 250 x + 5 right) $$
$$ left( x^{5} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{2} - 10 x + 75 right) + left( - 500 x^{2} - 1875 x + 5 right) $$
$$ left( x^{6} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{3} - 10 x^{2} + 75 x - 500 right) + left( 3125 x^{2} + 12500 x + 5 right) $$
$$ left( x^{7} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{4} - 10 x^{3} + 75 x^{2} - 500 x + 3125 right) + left( - 18750 x^{2} - 78125 x + 5 right) $$
$$ left( x^{8} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{5} - 10 x^{4} + 75 x^{3} - 500 x^{2} + 3125 x - 18750 right) + left( 109375 x^{2} + 468750 x + 5 right) $$
$$ left( x^{9} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{6} - 10 x^{5} + 75 x^{4} - 500 x^{3} + 3125 x^{2} - 18750 x + 109375 right) + left( - 625000 x^{2} - 2734375 x + 5 right) $$
$$ left( x^{10} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{7} - 10 x^{6} + 75 x^{5} - 500 x^{4} + 3125 x^{3} - 18750 x^{2} + 109375 x - 625000 right) + left( 3515625 x^{2} + 15625000 x + 5 right) $$
$endgroup$
add a comment |
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3 Answers
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$begingroup$
It is a good idea to use the roots of $q(x)=x^3+10x^2+25x$, but yes: because $-5$ is a double root, things get a little bit trickier. (Also, check that setting $x=0$ gives $C=5$, not $C=0$).
HINT: But since $-5$ is a double root of $q$, it is both a root of $q$ and $q'$. So try to take the derivative at both members and then take again $x=-5$.
$endgroup$
add a comment |
$begingroup$
It is a good idea to use the roots of $q(x)=x^3+10x^2+25x$, but yes: because $-5$ is a double root, things get a little bit trickier. (Also, check that setting $x=0$ gives $C=5$, not $C=0$).
HINT: But since $-5$ is a double root of $q$, it is both a root of $q$ and $q'$. So try to take the derivative at both members and then take again $x=-5$.
$endgroup$
add a comment |
$begingroup$
It is a good idea to use the roots of $q(x)=x^3+10x^2+25x$, but yes: because $-5$ is a double root, things get a little bit trickier. (Also, check that setting $x=0$ gives $C=5$, not $C=0$).
HINT: But since $-5$ is a double root of $q$, it is both a root of $q$ and $q'$. So try to take the derivative at both members and then take again $x=-5$.
$endgroup$
It is a good idea to use the roots of $q(x)=x^3+10x^2+25x$, but yes: because $-5$ is a double root, things get a little bit trickier. (Also, check that setting $x=0$ gives $C=5$, not $C=0$).
HINT: But since $-5$ is a double root of $q$, it is both a root of $q$ and $q'$. So try to take the derivative at both members and then take again $x=-5$.
edited Jan 29 at 20:31
answered Jan 29 at 20:00


Alejandro Nasif SalumAlejandro Nasif Salum
4,765118
4,765118
add a comment |
add a comment |
$begingroup$
Hint $ $ Subtracting $, C = 5,$ then cancelling $x$ yields
$qquad f(x) := x^{large n-1} = B + Ax + (x+5)^{large 2} p(x) $
Thus $, B + Ax = f(-5) + f'(-5)(x+5),$ by Taylor expansion at $,x = -5,$
$endgroup$
$begingroup$
If you don't know (or can't use) Taylor then you could instead apply this (purely algebraic) Polynomial Double Root Test. on $,x^{n-1}!-B-Ax,,$ or just differentiate as I hinted on the question.
$endgroup$
– Bill Dubuque
Jan 29 at 20:22
add a comment |
$begingroup$
Hint $ $ Subtracting $, C = 5,$ then cancelling $x$ yields
$qquad f(x) := x^{large n-1} = B + Ax + (x+5)^{large 2} p(x) $
Thus $, B + Ax = f(-5) + f'(-5)(x+5),$ by Taylor expansion at $,x = -5,$
$endgroup$
$begingroup$
If you don't know (or can't use) Taylor then you could instead apply this (purely algebraic) Polynomial Double Root Test. on $,x^{n-1}!-B-Ax,,$ or just differentiate as I hinted on the question.
$endgroup$
– Bill Dubuque
Jan 29 at 20:22
add a comment |
$begingroup$
Hint $ $ Subtracting $, C = 5,$ then cancelling $x$ yields
$qquad f(x) := x^{large n-1} = B + Ax + (x+5)^{large 2} p(x) $
Thus $, B + Ax = f(-5) + f'(-5)(x+5),$ by Taylor expansion at $,x = -5,$
$endgroup$
Hint $ $ Subtracting $, C = 5,$ then cancelling $x$ yields
$qquad f(x) := x^{large n-1} = B + Ax + (x+5)^{large 2} p(x) $
Thus $, B + Ax = f(-5) + f'(-5)(x+5),$ by Taylor expansion at $,x = -5,$
answered Jan 29 at 20:12
Bill DubuqueBill Dubuque
213k29196654
213k29196654
$begingroup$
If you don't know (or can't use) Taylor then you could instead apply this (purely algebraic) Polynomial Double Root Test. on $,x^{n-1}!-B-Ax,,$ or just differentiate as I hinted on the question.
$endgroup$
– Bill Dubuque
Jan 29 at 20:22
add a comment |
$begingroup$
If you don't know (or can't use) Taylor then you could instead apply this (purely algebraic) Polynomial Double Root Test. on $,x^{n-1}!-B-Ax,,$ or just differentiate as I hinted on the question.
$endgroup$
– Bill Dubuque
Jan 29 at 20:22
$begingroup$
If you don't know (or can't use) Taylor then you could instead apply this (purely algebraic) Polynomial Double Root Test. on $,x^{n-1}!-B-Ax,,$ or just differentiate as I hinted on the question.
$endgroup$
– Bill Dubuque
Jan 29 at 20:22
$begingroup$
If you don't know (or can't use) Taylor then you could instead apply this (purely algebraic) Polynomial Double Root Test. on $,x^{n-1}!-B-Ax,,$ or just differentiate as I hinted on the question.
$endgroup$
– Bill Dubuque
Jan 29 at 20:22
add a comment |
$begingroup$
I think there is an error in the attempt that is part of the question. Anyway, here are some relevant quotients and remainders:
$$ left( x^{4} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x - 10 right) + left( 75 x^{2} + 250 x + 5 right) $$
$$ left( x^{5} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{2} - 10 x + 75 right) + left( - 500 x^{2} - 1875 x + 5 right) $$
$$ left( x^{6} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{3} - 10 x^{2} + 75 x - 500 right) + left( 3125 x^{2} + 12500 x + 5 right) $$
$$ left( x^{7} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{4} - 10 x^{3} + 75 x^{2} - 500 x + 3125 right) + left( - 18750 x^{2} - 78125 x + 5 right) $$
$$ left( x^{8} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{5} - 10 x^{4} + 75 x^{3} - 500 x^{2} + 3125 x - 18750 right) + left( 109375 x^{2} + 468750 x + 5 right) $$
$$ left( x^{9} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{6} - 10 x^{5} + 75 x^{4} - 500 x^{3} + 3125 x^{2} - 18750 x + 109375 right) + left( - 625000 x^{2} - 2734375 x + 5 right) $$
$$ left( x^{10} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{7} - 10 x^{6} + 75 x^{5} - 500 x^{4} + 3125 x^{3} - 18750 x^{2} + 109375 x - 625000 right) + left( 3515625 x^{2} + 15625000 x + 5 right) $$
$endgroup$
add a comment |
$begingroup$
I think there is an error in the attempt that is part of the question. Anyway, here are some relevant quotients and remainders:
$$ left( x^{4} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x - 10 right) + left( 75 x^{2} + 250 x + 5 right) $$
$$ left( x^{5} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{2} - 10 x + 75 right) + left( - 500 x^{2} - 1875 x + 5 right) $$
$$ left( x^{6} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{3} - 10 x^{2} + 75 x - 500 right) + left( 3125 x^{2} + 12500 x + 5 right) $$
$$ left( x^{7} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{4} - 10 x^{3} + 75 x^{2} - 500 x + 3125 right) + left( - 18750 x^{2} - 78125 x + 5 right) $$
$$ left( x^{8} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{5} - 10 x^{4} + 75 x^{3} - 500 x^{2} + 3125 x - 18750 right) + left( 109375 x^{2} + 468750 x + 5 right) $$
$$ left( x^{9} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{6} - 10 x^{5} + 75 x^{4} - 500 x^{3} + 3125 x^{2} - 18750 x + 109375 right) + left( - 625000 x^{2} - 2734375 x + 5 right) $$
$$ left( x^{10} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{7} - 10 x^{6} + 75 x^{5} - 500 x^{4} + 3125 x^{3} - 18750 x^{2} + 109375 x - 625000 right) + left( 3515625 x^{2} + 15625000 x + 5 right) $$
$endgroup$
add a comment |
$begingroup$
I think there is an error in the attempt that is part of the question. Anyway, here are some relevant quotients and remainders:
$$ left( x^{4} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x - 10 right) + left( 75 x^{2} + 250 x + 5 right) $$
$$ left( x^{5} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{2} - 10 x + 75 right) + left( - 500 x^{2} - 1875 x + 5 right) $$
$$ left( x^{6} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{3} - 10 x^{2} + 75 x - 500 right) + left( 3125 x^{2} + 12500 x + 5 right) $$
$$ left( x^{7} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{4} - 10 x^{3} + 75 x^{2} - 500 x + 3125 right) + left( - 18750 x^{2} - 78125 x + 5 right) $$
$$ left( x^{8} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{5} - 10 x^{4} + 75 x^{3} - 500 x^{2} + 3125 x - 18750 right) + left( 109375 x^{2} + 468750 x + 5 right) $$
$$ left( x^{9} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{6} - 10 x^{5} + 75 x^{4} - 500 x^{3} + 3125 x^{2} - 18750 x + 109375 right) + left( - 625000 x^{2} - 2734375 x + 5 right) $$
$$ left( x^{10} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{7} - 10 x^{6} + 75 x^{5} - 500 x^{4} + 3125 x^{3} - 18750 x^{2} + 109375 x - 625000 right) + left( 3515625 x^{2} + 15625000 x + 5 right) $$
$endgroup$
I think there is an error in the attempt that is part of the question. Anyway, here are some relevant quotients and remainders:
$$ left( x^{4} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x - 10 right) + left( 75 x^{2} + 250 x + 5 right) $$
$$ left( x^{5} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{2} - 10 x + 75 right) + left( - 500 x^{2} - 1875 x + 5 right) $$
$$ left( x^{6} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{3} - 10 x^{2} + 75 x - 500 right) + left( 3125 x^{2} + 12500 x + 5 right) $$
$$ left( x^{7} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{4} - 10 x^{3} + 75 x^{2} - 500 x + 3125 right) + left( - 18750 x^{2} - 78125 x + 5 right) $$
$$ left( x^{8} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{5} - 10 x^{4} + 75 x^{3} - 500 x^{2} + 3125 x - 18750 right) + left( 109375 x^{2} + 468750 x + 5 right) $$
$$ left( x^{9} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{6} - 10 x^{5} + 75 x^{4} - 500 x^{3} + 3125 x^{2} - 18750 x + 109375 right) + left( - 625000 x^{2} - 2734375 x + 5 right) $$
$$ left( x^{10} + 5 right) = left( x^{3} + 10 x^{2} + 25 x right) cdot left( x^{7} - 10 x^{6} + 75 x^{5} - 500 x^{4} + 3125 x^{3} - 18750 x^{2} + 109375 x - 625000 right) + left( 3515625 x^{2} + 15625000 x + 5 right) $$
edited Jan 30 at 0:08
answered Jan 29 at 21:51
Will JagyWill Jagy
104k5102201
104k5102201
add a comment |
add a comment |
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$begingroup$
Don't you get $C=5$ by setting $x=0$?
$endgroup$
– Alejandro Nasif Salum
Jan 29 at 19:54
1
$begingroup$
Hint: differentiate, since $,x = -5,$ is a double root you can again evaluate at $,x = -5,$ to get another equation in $A,B$
$endgroup$
– Bill Dubuque
Jan 29 at 19:58