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The polynomials 𝑥3+𝑎 𝑥2−𝑥+𝑏x 3 +a x 2 −x+b and 𝑥3+𝑏 𝑥2−5 𝑥+3 𝑎x 3 +b x 2 −5 x+3 a both have 𝑥+2x+2 as a factor, the values of 𝑎a and 𝑏b are

Question

The polynomials

x3+ax2x+b x^3 + a x^2 - x + b
and
x3+bx25x+3a x^3 + b x^2 - 5 x + 3 a
both have x+2 x + 2 as a factor, the values of a a and b b are

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Solution

Given that both polynomials have x+2 as a factor, we can use the Factor Theorem which states that a polynomial f(x) has a factor (x-c) if and only if f(c) = 0.

For the first polynomial, x^3 + ax^2 - x + b, we substitute x = -2:

(-2)^3 + a(-2)^2 - (-2) + b = 0 -8 + 4a + 2 + b = 0 4a + b = 6 ----(1)

For the second polynomial, x^3 + bx^2 - 5x + 3a, we substitute x = -2:

(-2)^3 + b(-2)^2 - 5(-2) + 3a = 0 -8 + 4b + 10 + 3a = 0 4b + 3a = -2 ----(2)

Now we have a system of linear equations. We can solve this system to find the values of a and b.

Multiplying equation (1) by 3 and equation (2) by 4 gives:

12a + 3b = 18 ----(3) 16b + 12a = -8 ----(4)

Subtracting equation (4) from equation (3) gives:

-4b = 26 b = -26/4 b = -6.5

Substituting b = -6.5 into equation (1) gives:

4a - 6.5 = 6 4a = 12.5 a = 12.5 / 4 a = 3.125

So, the values of a and b are 3.125 and -6.5 respectively.

This problem has been solved

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