Stubborn polynomial

Algebra Level 2

When divided by x 1 x-1 the polynomial p ( x ) p(x) leaves a remainder 2 2 .When divided by x 2 x-2 leaves a remainder 5.When divided by ( x 1 ) ( x 2 ) (x-1)(x-2) leaves a remainder of the form a x + b ax+b . Evaluate a a .


The answer is 3.

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13 solutions

Shivam Khosla
Dec 23, 2013

P(x)=(x-1)q+2.........(1)
P(x)=(x-2)w+5.........(2)
P(x)=(x-1)(x-2)t+ax+b......(3). where q,w,t are quotients
Putting x=1 in 1 we get
P(1)=2,
Putting x=2 in 2
P(2)=5
Putting x=1 in 3
P(1)=a+b
Putting x=2 in 3
P(2)=2a+b...
Solving all this we get a=3


awesome...

Amlan Mishra - 7 years, 4 months ago

I solve it the same way as you did...

Aman Bansal - 7 years, 3 months ago
Lorenc Bushi
Dec 22, 2013

From the question , we have: p ( x ) x 1 = f ( x ) + 2 x 1 \frac{p(x)}{x-1}=f(x)+\frac{2}{x-1} ( 1 ) (1) p ( x ) x 2 = h ( x ) + 5 x 2 \frac{p(x)}{x-2}=h(x)+\frac{5}{x-2} ( 2 ) (2)
Substracting ( 1 ) (1) from ( 2 ) (2) ,we obtain: p ( x ) ( x 1 ) ( x 2 ) = h ( x ) f ( x ) + 3 x 1 ( x 1 ) ( x 2 ) p ( x ) = ( h ( x ) f ( x ) ) ( x 1 ) ( x 2 ) + 3 x 1 \frac{p(x)}{(x-1)(x-2)}=h(x)-f(x)+\frac{3x-1}{(x-1)(x-2)} p(x)=(h(x)-f(x))(x-1)(x-2)+3x-1 ,therefore the remainder is 3 x 1 3x-1 , where we obtain a = 3 a=3

Respectively, let q ( x ) , r ( x ) q(x), r(x) and s ( x ) s(x) be the quotient when p ( x ) p(x) is divided by ( x 1 ) (x-1) , ( x 2 ) (x-2) , and ( x 1 ) ( x 2 ) (x-1)(x-2) . Hence we can rewrite p ( x ) p(x) as below:

p ( x ) = ( x 1 ) q ( x ) + 2... ( 1 ) p(x)=(x-1)q(x)+2...(1)

p ( x ) = ( x 2 ) r ( x ) + 5... ( 2 ) p(x)=(x-2)r(x)+5...(2)

p ( x ) = ( x 1 ) ( x 2 ) s ( x ) + a x + b . . . ( 3 ) p(x)=(x-1)(x-2)s(x)+ax+b...(3)

From (1) we have p ( 1 ) = ( 1 1 ) q ( x ) + 2 = 0 + 2 = 2 p(1)=(1-1)q(x)+2=0+2=2

Similarly, from (2) we have p ( 2 ) = 5 p(2)=5

Thus p ( 1 ) p ( 2 ) p(1)\ne p(2) . But from (3) we have p ( 1 ) = p ( 2 ) = a x + b p(1)=p(2)=ax+b

Indirectly, it is stated that p ( 1 ) p ( 2 ) p(1)\ne p(2) and p ( 1 ) = p ( 2 ) p(1) = p(2) at once

This is impossible unless q ( x ) = r ( x ) = s ( x ) = 0 q(x)=r(x)=s(x)=0 , such that it results the new conclution: p ( x ) = a x + b p(x)=ax+b .

Therefore, now we have p ( 1 ) = a + b = 2 p(1)=a+b=2 and p ( 2 ) = 2 a + b = 5 p(2)=2a+b=5 . And p ( 2 ) p ( 1 ) = a = 3 p(2)-p(1)=a=\boxed {3}

Prathyusha Konda
Dec 23, 2013

p(x) = (x-1)(x-2) f(x) + (ax+b) where f(x) is quotient and ax+b is the remainder. On applying the first two conditions, i.e., when x=1 p(x)=2 and x=2 p(x)=5; a+b = 2 and 2a+b=5. Solving the above two equations, we get a=3 and b=-1

Budi Utomo
Dec 23, 2013

2 = a + b , 5 = 2a + b, So, with substitution method we have b = -1 and a = 3. Answer : 3

You should elaborate on your solution.

Tilak Patel - 7 years, 5 months ago
Jackie Nguyen
Dec 23, 2013

From the given statement, we can express that

P(x) = (x-1) Q(x) + 2 => P(1) = 2 P(x) = (x-2) K(x) + 5 => P(2) = 5

Finally: P(x) = (x-1)(x-2) H(x) + ax + b Replace x by 1 and 2, we have a + b = 2 2a + b = 5 => a = 3

Can admin fix my post :( It's hard to read :(

Jackie Nguyen - 7 years, 5 months ago
Bhargav Das
Dec 23, 2013

It follows easily from Chinese Remainder Theorem .We get: a x + b = 3 x 2 ax+b=3x-2 giving a = 3 a=\boxed{3} .

Can you please explain that what is chinese remainder theoram??

kirtan bhatt - 7 years, 5 months ago

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The Chinese Remainder Theorem has nearly nothing to do with the algebra of polynomials, as far as I know. It's a result about modular congruences in number theory.

Michael Tang - 7 years, 5 months ago

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This is true. However, we can easily generalize the CRT to any commutative ring. The method would not be as easy as Bhargav claims, unless Bhargav is using something clever that he's not saying, but it would get you p ( x ) p(x) by using a fairly simple algorithm.

Jacob Erickson - 7 years, 5 months ago

Yes, historically this was a theorem about the integers. However, almost everything that works for numbers, works for polynomials, including, as Jacob Erickson noted, the Chinese Remainder Theorem. My favorite example of this kind is the construction of complex numbers as real polynomials modulo the irreducible polynomial x 2 + 1 x^2+1 .

It does not quite work the other way. For example, the arithmetic analog of the Mason-Stothers theorem is the notoriously hard abc-conjecture.

Alexander Borisov - 7 years, 5 months ago

I actually became a bit of a lethargic that day.So,let me clear things up now. Here goes:

STEP 1 : A/Q we have, p ( x ) 2 ( m o d ( x 1 ) ) p(x)\equiv 2 \pmod{(x-1)}

and p ( x ) 5 ( m o d ( x 2 ) ) p(x)\equiv 5 \pmod{(x-2)}

Let b 1 b_1 and b 2 b_2 be 2 2 and 5 5 respectively.

We need to find: p ( x ) ( m o d ( x 1 ) ( x 2 ) ) p(x)\equiv * \pmod{(x-1)(x-2)} ,where * is of the form a x + b ax+b and where a a and b b are integers.

STEP 2 : Let M = ( x 1 ) ( x 2 ) M=(x-1)(x-2) .

Hence, M 1 = M x 1 = ( x 2 ) M_1=\frac {M}{x-1}=(x-2) and M 2 = M x 2 = ( x 1 ) M_2=\frac {M}{x-2}=(x-1) .

STEP 3 : We need to find x 1 x_1 in: M 1 x 1 1 ( m o d ( x 1 ) ) M_1x_1\equiv 1 \pmod{(x-1)} .

x 1 = 1 \implies x_1=-1 (Self-explanatory).

Similarly, We need to find x 2 x_2 in: M 2 x 2 1 ( m o d ( x 1 ) ) M_2x_2\equiv 1 \pmod{(x-1)} .

x 2 = 1 \implies x_2=1 (Self-explanatory).

STEP 4 : X = b 1 M 1 x 1 + b 2 M 2 x 2 + b 3 M 3 x 3 = 2 ( x 2 ) ( 1 ) + 5 ( x 1 ) 1 = 3 x 1 X=b_1M_1x_1+b_2M_2x_2+b_3M_3x_3=2 \cdot (x-2) \cdot (-1)+5 \cdot (x-1) \cdot 1= 3x-1

X = b 1 M 1 x 1 + b 2 M 2 x 2 + b 3 M 3 x 3 = 3 x 1 ( 3 x 1 ) ( m o d ( x 1 ) ( x 2 ) ) X=b_1M_1x_1+b_2M_2x_2+b_3M_3x_3=3x-1 \equiv (3x-1) \pmod{(x-1)(x-2)} .

Hence, a = 3 . \implies a=\boxed{3}.

One can visit rather download a .ppt extension file from here for more info about CHINESE REMAINDER THEOREM .

Bhargav Das - 7 years, 5 months ago

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It is, as I said, a very simple algorithm. I would not consider that "easy," though.

All the same, nicely done. :D

Jacob Erickson - 7 years, 5 months ago

Sorry,a typo: It should be 3 x 1 3x-1 .

Bhargav Das - 7 years, 5 months ago
Kushagra Sahni
Feb 7, 2014

p(x)=(x-1)Q1(x)+2 p(x)=(x-2)Q2(x)+5 p(x)=(x-1)(x-2)Q3(x)+ax+b

p(1)=2 p(1)=a+b p(2)=5 p(2)=2a+b

Hence, a+b=2 -a-b=-2 2a-a-b+b=-2+5 a=3, b=-1
2a+b=5 2a+b=5 a+b=2, 3+b=2, b=2-3=-1
Hence, the remainder will be ax+b=3x-1

Ans: 3x-1

Karandeep Singh
Jan 16, 2014

by Division Algorithm,

Dividend=Divisor*quotient+remainder

So,p(x)=(x-1)*q(x)+2

putting x=1,

p(1)=0+2=2 -(i)

Also, p(x)=(x-2)*q'(x)+5

so, p(2)=5 -(ii)

Now, p(x)=(x-1)(x-2)*Q(x)+(ax+b)

putting x=1 and x=2

p(1)=a+b and p(2)=2a+b

so, a+b=2 and 2a+b=5 [from (i) and (ii)]

solving the two, we get a =3

Jeremi Litarowicz
Jan 16, 2014

We first consder the case in which ( x 1 ) ( x 2 ) = ( x 1 ) (x-1)(x-2)=(x-1) or x = 3 x=3 . In this case, we have that 2 = 3 a + b 2=3a+b . Now, let's consider the case in which ( x 1 ) ( x 2 ) = 2 ( x 1 ) x = 4 (x-1)(x-2)=2(x-1) \Rightarrow x=4 . Note that if n m ( m o d p ) n \equiv m \pmod{p} , then n m ( m o d 2 p ) n \equiv m \pmod{2p} or n m + p ( m o d 2 p ) n \equiv m+p \pmod{2p} . We thus get that 2 = 4 a + b 2=4a+b or 5 = 4 a + b 5=4a+b . If we use the first equation, a a solves to zero, and that's not the case, so we have to use the second equation. This gives us a = 3 \boxed{a=3} .

Sujay Jadhav
Jan 14, 2014

Let p(x)=(x-1)(x-2)Q(x)+ax+b dividend=divisor multiplied by quotient plus remainder. Given p(1)=2=a+b and p(2)=5=2a+b by the remainder theorem. Solving, a=3

Pankaj Joshi
Jan 4, 2014

Let P(x) be

(x-1)(x-2).Q(x) + r(x) where r(x) is Ax + B

so, P(1) = A+B = 2

and, P(2)= 2A+B = 5

On solving we get, A= 3 and B=-1

Hence the answer is 3 \boxed{3}

Akhmad Dainuri
Dec 24, 2013

f(x) = (x-1).g(x) + 2 ; f(1) = 2

f(x) = (x-2).h(x) + 5 ;f(2) = 5

f(x) = (x-1).(x-2).s(x) + ax + b

f(1) = a + b = 2 . ......(1)

f(2) = 2a + b = 5 ........(2)

then, by eliminating equation (1) and (2), we get that a is 3

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