Divisible or not?

Level pending

Find a a such that x 2 x-2 is a factor of : P ( x ) = 2 a x 3 4 x 2 + a x 2 a P(x)=2ax^3-4x^2+ax-2a .


The answer is 1.

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2 solutions

Prasun Biswas
Jan 4, 2014

First, I should state the Remainder Theorem. According to it if ( x b ) (x-b) divides a polynomial P(x), then P(b)=Remainder after division and if ( x b ) (x-b) is a factor of a polynomial P(x), then P(b)=0 since then there is no remainder.

So, the polynomial P(x) here is divided by ( x 2 ) (x-2) , so P(2)=0

P ( 2 ) = 0 P(2)=0

2 a × 2 3 4 × 2 2 + 2 a 2 a = 0 \implies 2a\times 2^{3} - 4\times 2^{2} + 2a - 2a=0

2 a × 8 4 × 4 = 0 \implies 2a\times 8 - 4\times 4=0

16 a 16 = 0 16 a = 16 a = 1 \implies 16a-16=0 \implies 16a=16 \implies a=\boxed{1}

Sanghamitra Anand
Dec 30, 2013

P(x) is the remainder of the polynomial. P(x) = 0, since (x - 2) is a factor of the polynomial 2ax³ - 4x² + ax - 2a.
To find the value of x, we have to find the root of the polynomial factor -- (x - 2).

Assume that x - 2 = 0.
Therefore, x = 2.
Now substitute the value of x in the equation to find 'a': P(x) = 2ax³ - 4x² + ax - 2a
P(2) = 2a(2)³ - 4(2)² + a(2) - 2a
0 = 2a(8) - 4(4) + 2a - 2a
0 = 16a - 16
16 = 16a
16a = 16
a = 16/16
Therefore, a = 1.


You should have said that you used here the Remainder Theorem. Also, how can you simply say that P(x) is the remainder of the polynomial P(x) ??

Prasun Biswas - 7 years, 5 months ago

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Sorry, I must have made a mistake then. since, (x - 2) is a factor, the remainder must be zero. I shouldn't have included the first statement! Thank you :)

Sanghamitra Anand - 7 years, 5 months ago

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