Find the remainder when 6x3−13x2+17x−66x^3 - 13x^2 + 17x - 6 is divided by 2x−12x - 1. Show that there is only one real root of the equation
6x3−13x2+17x−6=0.6x^3 - 13x^2 + 17x - 6 = 0.

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Worked solution

4(a)

1. Apply the Remainder Theorem by substituting $x = \frac{1}{2}$ into the polynomial

Let f(x)=6x3−13x2+17x−6f(x) = 6x^3 - 13x^2 + 17x - 6.
By the Remainder Theorem, the remainder when f(x)f(x) is divided by 2x−12x - 1 is f(12)f\left(\frac{1}{2}\right).
f(12)=6(12)3−13(12)2+17(12)−6f\left(\frac{1}{2}\right) = 6\left(\frac{1}{2}\right)^3 - 13\left(\frac{1}{2}\right)^2 + 17\left(\frac{1}{2}\right) - 6

2. Evaluate the expression to find the remainder

f(12)=6(18)−13(14)+172−6f\left(\frac{1}{2}\right) = 6\left(\frac{1}{8}\right) - 13\left(\frac{1}{4}\right) + \frac{17}{2} - 6
=34−134+344−244= \frac{3}{4} - \frac{13}{4} + \frac{34}{4} - \frac{24}{4}
=3−13+34−244= \frac{3 - 13 + 34 - 24}{4}
=0= 0

Answer

00

4(b)

1. Use the result from part (a) to factorise the cubic polynomial

From part (a), P(12)=6(18)−13(14)+17(12)−6=0P\left(\frac{1}{2}\right) = 6\left(\frac{1}{8}\right) - 13\left(\frac{1}{4}\right) + 17\left(\frac{1}{2}\right) - 6 = 0, so 2x−12x - 1 is a factor.
By long division or comparing coefficients:
6x3−13x2+17x−6=(2x−1)(3x2−5x+6)6x^3 - 13x^2 + 17x - 6 = (2x - 1)(3x^2 - 5x + 6)

2. Find the real root from the linear factor

For (2x−1)(3x2−5x+6)=0(2x - 1)(3x^2 - 5x + 6) = 0,
2x−1=0  ⟹  x=122x - 1 = 0 \implies x = \frac{1}{2}

3. Show that the quadratic factor has no real roots using the discriminant

For the quadratic factor 3x2−5x+6=03x^2 - 5x + 6 = 0,
Discriminant Δ=b2−4ac=(−5)2−4(3)(6)\Delta = b^2 - 4ac = (-5)^2 - 4(3)(6)
Δ=25−72=−47\Delta = 25 - 72 = -47
Since Δ<0\Delta < 0, the quadratic equation 3x2−5x+6=03x^2 - 5x + 6 = 0 has no real roots.
Therefore, x=12x = \frac{1}{2} is the only real root.

Answer

Since the discriminant of the quadratic factor is negative, it has no real roots. Thus, x=12x = \frac{1}{2} is the only real root.

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