They are just powers of 3

Algebra Level 3

What is the remainder when the polynomial x + x 3 + x 9 + x 27 + x 81 + x 243 x+x^{3}+x^{9}+x^{27}+x^{81}+x^{243} is divided by x 2 1 x^2-1 ?

NMTC problem.
6 x 6x None of these choices 1 1 2 x 2x 4 x 4x

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1 solution

Akshat Sharda
Jun 22, 2015

We know , y 1 y-1 divides y n 1 y^{n}-1 for all positive integers n n . Replacing y y by x 2 x^{2} we see that, x 2 x^{2} divides x 2 n 1 x^{2n}-1 for all positive integers n n .

Therefore , x 243 + x 81 + x 27 + x 9 + x 3 + x x^{243}+x^{81}+x^{27}+x^{9}+x^{3}+x

= x 243 x + x 81 x + x 27 x + x 9 x 3 x + 5 x + x =x^{243}-x+x^{81}-x+x^{27}-x+x^{9}-x^{3}-x+5x+x

= x ( x 242 1 ) + x ( x 80 1 ) + x ( x 26 1 ) + x ( x 8 1 ) + x ( x 2 1 ) + 6 x =x(x^{242}-1)+x(x^{80}-1)+x(x^{26}-1)+x(x^{8}-1)+x(x^{2}-1)+6x

Now , ( x 242 1 ) , ( x 80 1 ) , ( x 26 1 ) , ( x 8 1 ) (x^{242}-1),(x^{80}-1),(x^{26}-1),(x^{8}-1) and x ( x 2 1 ) x(x^{2}-1) are exactly divisible by ( x 2 1 ) (x^{2}-1) .

Therefore , 6 x \boxed{6x} is the remainder.

cant we use the remainder theorem?

gahin maiti - 5 years, 11 months ago

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We can use remainder theorem

Akshat Sharda - 5 years, 9 months ago

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thanks.actually I solved it by that so I asked it.

gahin maiti - 5 years, 8 months ago

I'll post its solution when we solve it by remainder theorem soon.

Akshat Sharda - 5 years, 9 months ago

What if x = 1 or -1? x ^ 2 - 1 = 0. So 6 divided by 0 is undefined!

Thomas Xin - 5 years, 11 months ago

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