Exists or not ....

Algebra Level 3

Suppose that there exists a polynomial which when divided by (x-8) gives a remainder 2, and when it is divided by (x+2) gives a remainder 3.Find the remainder when that polynomial will be divided by (x-8) (x+2)

-14/5 x +1/10 14/5 x +1/10 -1/10 x +14/5 1/10 x +14/5

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1 solution

Let the polynomial be f ( x ) f(x) .

By the remainder theorem, when f ( x ) f(x) is divided by ( x a ) (x-a) , f ( a ) = r ( a ) f(a) = r(a) , where r ( x ) r(x) is the remainder.

Then, by the conditions given in the condition, we have,

When f ( x ) f(x) is divided by ( x 8 ) (x-8) , f ( 8 ) = 2 \Rightarrow f(8) = 2 .

When f ( x ) f(x) is divided by ( x + 2 ) (x+2) , f ( 2 ) = 3 \Rightarrow f(-2) = 3

When f ( x ) f(x) is divided by ( x 8 ) ( x + 2 ) (x-8)(x+2) the divisor has degree 2. Therefore The remainder has degree 1 or less. Let r ( x ) = A x + B r(x) = Ax + B .

f ( 8 ) = r ( 8 ) = A ( 8 ) + B f(8) = r(8) = A(8) + B

2 = 8 A + B 2 = 8A + B

f ( 2 ) = r ( 2 ) = A ( 2 ) + B f(-2) = r(-2) = A(-2) + B

3 = 2 A + B 3 = -2A + B

Solving these simultaneous equations gives A = 1 10 , b = 14 5 A = \frac{-1}{10}, b= \frac{14}{5} and r ( x ) = 1 10 x + 14 5 r(x) = \frac{-1}{10}x + \frac{14}{5}

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