1. ## Cubic Eqations

1. Determine the remainder when 9x5 – 4x4 is divided by 3x-1

2. Show, using factor theorem, that 2x-1 is a factor of:

2x4 - x3-6x2 + 5x -1
and hence express 2x4 - x3-6x2 + 5x -1 as a product of a linear and cubic factor

Where do I start?

2. 1. Determine the remainder when 9x5 – 4x4 is divided by 3x-1
You can use either the polynomial long division, or the polynomial remainder theorem. According to the latter, the remainder of f(x) divided by k(x-a) is f(a).

2. Show, using factor theorem, that 2x-1 is a factor of:

2x4 - x3-6x2 + 5x -1
and hence express 2x4 - x3-6x2 + 5x -1 as a product of a linear and cubic factor
According to the polynomial remainder theorem above (in this case it is the same as the factor theorem), it is sufficient to represent 2x-1 as $k(x - a)$ for some $k$ and $a$ and then to show that $a$ is the root of the given polynomial. To find the cubic factor, use the long division to divide the given fourth-degree polynomial by 2x - 1.

3. Originally Posted by DanBrown100
1. Determine the remainder when 9x5 – 4x4 is divided by 3x-1

2. Show, using factor theorem, that 2x-1 is a factor of:

2x4 - x3-6x2 + 5x -1
and hence express 2x4 - x3-6x2 + 5x -1 as a product of a linear and cubic factor

Where do I start?
1. $\displaystyle f(x) = 9x^5 - 4x^4$.

The remainder when divided by $\displaystyle 3x - 1$ is given by $\displaystyle f\left(\frac{1}{3}\right)$.

2. $\displaystyle f(x) = 2x^4 - x^3 - 6x^2 + 5x - 1$.

To show that $\displaystyle 2x - 1$ is a factor, check that $\displaystyle f\left(\frac{1}{2}\right) = 0$.

Then you need to use long division to express $\displaystyle f(x)$ as $\displaystyle (2x-1)Q(x)$.