Remainder!

Algebra Level 4

Find the remainder when the polynomial x 81 + x 49 + x 25 + x 9 + x x^{81}+x^{49}+x^{25} + x^9+x is divided by x 3 x 2 x^3-x^2 .

x 2 + x x^{2} + x 4 x 2 + x 4x^{2} + x 3 x 2 + x 3x^{2} + x 7 x 2 + x 7x^{2} + x 5 x 2 + x 5x^{2} + x 9 x 2 + x 9x^{2} + x

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4 solutions

Shivram Badhe
Jan 30, 2015

Aareyan Manzoor
Feb 4, 2015

let P ( x ) = x 3 x 2 , F ( x ) = x 81 + x 49 + x 25 + x 9 + x P(x)=x^3 -x^2, F(x)=x^{81}+x^{49}+x^{25}+x^9+x and r ( x ) = r e m a i n d e r r(x) =remainder than F ( x ) = P ( x ) Q ( x ) + r ( x ) F(x)=P(x)Q(x)+r(x) for some polynomial Q(x) F ( x ) r ( x ) = P ( x ) Q ( x ) F(x)-r(x)=P(x)Q(x) and hence P ( x ) F ( x ) r ( x ) P(x)|F(x)-r(x) that also means that if P ( a 1 ) = P ( a 2 ) = 0 , F ( a 1 ) r ( a 1 ) = F ( a 2 ) r ( a 2 ) = 0 P(a_1)=P(a_2)=0, F(a_1)-r(a_1)=F(a_2)-r(a_2)=0 we see that the roots of P ( x ) = 0 , x = ( 1 , 0 ) P(x)=0, x =(1,0) hence F ( 1 ) r ( 1 ) = 0 r ( 1 ) = 5 F(1)-r(1)=0\longrightarrow r(1)=5 and F ( 0 ) r ( 0 ) = 0 r ( 0 ) = 0 F(0)-r(0)=0\longrightarrow r(0)=0 since zero is a root of r(x) and the its maximum degree is 2, it can be written as r ( x ) = ( x + 0 ) ( z x + n ) r(x)=(x+0)(zx+n) from 1 of the eqs, r ( 1 ) = 5 = 1 ( z + n ) > z + n = 5 r(1)=5=1(z+n)--> z+n=5 we see that the only choice from the option satisfying this is 4 x 2 + x \boxed{4x^2+x}

Naheem Ahmed
Jan 31, 2015

Substitute 1 into both equations. Observe the remainder is 5. Therefore the only answer is 4xsqaured +x

short and sweet solution !!! :-))

Shivram Badhe - 6 years, 4 months ago

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Iff we substitute x = 1 x=1 in the equation that you have typed in the fifth line of your solution we get 5 = a + b + c 5=a+b+c i.e sum of coefficients of the remainder is 5 Please point if wrong.

Chirayu Bhardwaj - 5 years, 2 months ago
Akhilesh Vibhute
Dec 11, 2015

at x=1 the GE=5 and also 4x^2+x=5 at x=1 simple!

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