The cubic polynomial is such that the coefficient of is and the roots of are , and . It is given that has a remainder of when divided by .
- (i)
Show that .
- (ii)
Hence find a value for and explain that there are no other real values of which satisfy this equation.
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Worked solution
2(i)
1. Formulate the expression for the cubic polynomial $f(x)$ using its roots and leading coefficient.
Since the roots are , , and , and the coefficient of is , we can write:
2. Apply the Remainder Theorem to set up an equation.
By the Remainder Theorem, since has a remainder of when divided by , we have .
3. Expand the equation and simplify to obtain the required result.
Shown.
2(ii)
1. Find an integer root of the cubic equation using the Factor Theorem
Let .
Try :
So is a root, and is a factor.
2. Factorise the cubic polynomial completely
Comparing coefficients of :
So,
3. State the real value of k
From the linear factor, .
4. Show that the quadratic factor has no real roots
For the quadratic factor , we check the discriminant:
Since , the equation has no real roots. Therefore, is the only real value.
There are no other real roots because the discriminant of the remaining quadratic factor is negative.
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