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Question

Let P(x)=1+x+x2+x3+x4+x5. What is the remainder when P(x12) is divided by P(x)?

A
0
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B
6
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C
1+x
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D
1+x+x2+x3+x4
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Solution

The correct option is A 6
As P(x) is the sum of GP. = 1x61x

It has 5 roots , let a1,a2,a3,a4,a5, and they are the 6th roots of unity except unity.

NowP(x12)=1+x12+x24+x36+x48+x60=P(x).Q(x)+R(x).

Here R(x) is a remainder and a polynomial of maximum degree 4.

Put x=a1,a2...............,a5
We get,
R(a1)=6, R(a2)=6 ,R(a3)=6, R(a4)=6, R(a5)=6

i.e, R(x)6=0 has 6 roots.

Which contradict that R(x) is maximum of degree 4.

So, it is an identity
Therefore, R(x)=6.

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