Question

consider a polynomial f(x)=ax^3 + bx^2 + x + 2/3.if x + 3 is a factor of f(x) and f(x) is divided by x + 2, then we get remainder as 5. find the values of a and b.

Answer 1

a = 2/27

b = 13/27

Explanation:

The given polynomial is presented as follows;

f(x) = a·x³ + b·x² + x + 2/3

Given that x + 3 is a factor, we have;

f(-3) = 0 = a·(-3)³ + b·(-3)² - 3 +2/3 = 0

-27·a + 9·b - 3 + 2/3 = 0

-27·a + 9·b = 7/3........(1)

Also we have

(a·x³ + b·x² + x + 2/3) ÷ (x + 2) the remainder = 5

Therefore;

a·(-2)³ + b·(-2)² + (-2) + 2/3 = 5

-8·a + 4·b - 2 + 2/3 = 5

-8·a + 4·b = 2 - 2/3 = 4/3........(2)

Multiplying equation (1) by 4/9 and subtracting it from equation (2), we have;

-8·a + 4·b - 4/9×(-27·a + 9·b) = 4/3 - 4/9 × 7/3

-8·a + 12·a = 8/27

4·a = 8/27

a = 2/27 ≈ 0.0741

imputing the a value in equation (1) gives;

-27×2/27 + 9·b = 7/3

-2 + 9·b = 7/3

9·b = 7/3 + 2 = 13/3

b = 13/27 ≈ 0.481.