The equation of a polynomial is f(x)=2x3−7x2+9x−3\text{f}(x) = 2x^3 - 7x^2 + 9x - 3.

  1. (i)

    Factorise the polynomial f(x)\text{f}(x).

  2. (ii)

    Determine the nature of the roots of the equation f(x)=0\text{f}(x) = 0.

  3. (iii)

    Solve the equation 2(33y)−7(32y)+3y+2=32(3^{3y}) - 7(3^{2y}) + 3^{y+2} = 3.

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Worked solution

4(i)

1. Find a linear factor of the polynomial using the Factor Theorem.

Let f(x)=2x3−7x2+9x−3\text{f}(x) = 2x^3 - 7x^2 + 9x - 3.
Try x=1x = 1:
f(1)=2(1)3−7(1)2+9(1)−3=2−7+9−3=0\text{f}(1) = 2(1)^3 - 7(1)^2 + 9(1) - 3 = 2 - 7 + 9 - 3 = 0.
Since f(1)=0\text{f}(1) = 0, (x−1)(x - 1) is a factor.

2. Divide the polynomial by the linear factor to find the quadratic factor.

Using long division or comparing coefficients:
2x3−7x2+9x−3=(x−1)(ax2+bx+c)2x^3 - 7x^2 + 9x - 3 = (x - 1)(ax^2 + bx + c)
Comparing x3x^3 coefficients: a=2a = 2
Comparing constant terms: −c=−3  ⟹  c=3-c = -3 \implies c = 3
Comparing x2x^2 coefficients: b−a=−7  ⟹  b−2=−7  ⟹  b=−5b - a = -7 \implies b - 2 = -7 \implies b = -5
So, f(x)=(x−1)(2x2−5x+3)\text{f}(x) = (x - 1)(2x^2 - 5x + 3).

3. Factorise the quadratic factor completely.

2x2−5x+3=(2x−3)(x−1)2x^2 - 5x + 3 = (2x - 3)(x - 1)
Therefore, f(x)=(x−1)(2x−3)(x−1)=(x−1)2(2x−3)\text{f}(x) = (x - 1)(2x - 3)(x - 1) = (x - 1)^2(2x - 3).

Answer

(x−1)2(2x−3)(x - 1)^2(2x - 3)

4(ii)

1. Use the factorised form of $\text{f}(x)$ from part (i) to set up the equation

From part (i), f(x)=(2x−1)(x2−3x+3)\text{f}(x) = (2x - 1)(x^2 - 3x + 3).
For f(x)=0\text{f}(x) = 0, we have:
(2x−1)(x2−3x+3)=0(2x - 1)(x^2 - 3x + 3) = 0

2. Identify the real root from the linear factor

2x−1=0  ⟹  x=122x - 1 = 0 \implies x = \frac{1}{2}
This gives one real root.

3. Analyse the quadratic factor using the discriminant

For the quadratic factor x2−3x+3=0x^2 - 3x + 3 = 0, we check the discriminant Δ\Delta:
Δ=b2−4ac=(−3)2−4(1)(3)\Delta = b^2 - 4ac = (-3)^2 - 4(1)(3)
Δ=9−12=−3\Delta = 9 - 12 = -3
Since Δ<0\Delta < 0, the quadratic equation x2−3x+3=0x^2 - 3x + 3 = 0 has no real roots.

4. State the final conclusion about the nature of the roots

Therefore, the equation f(x)=0\text{f}(x) = 0 has exactly one real root.

Answer

The equation has exactly one real root.

4(iii)

1. Rewrite the given equation to reveal the polynomial structure from part (i)

2(3y)3−7(3y)2+32(3y)−3=02(3^y)^3 - 7(3^y)^2 + 3^2(3^y) - 3 = 0
2(3y)3−7(3y)2+9(3y)−3=02(3^y)^3 - 7(3^y)^2 + 9(3^y) - 3 = 0

2. Substitute $x = 3^y$ and use the roots of $f(x) = 0$ from earlier parts

Let x=3yx = 3^y. The equation becomes f(x)=0f(x) = 0, which is 2x3−7x2+9x−3=02x^3 - 7x^2 + 9x - 3 = 0.
From part (i), f(x)=(x−1)2(2x−3)f(x) = (x-1)^2(2x-3).
So the roots are x=1x = 1 and x=32x = \frac{3}{2}.
Thus, 3y=13^y = 1 or 3y=1.53^y = 1.5.

3. Solve for $y$ using logarithms

For 3y=13^y = 1:
y=0y = 0

For 3y=1.53^y = 1.5:
ln⁡(3y)=ln⁡(1.5)\ln(3^y) = \ln(1.5)
yln⁡3=ln⁡1.5y \ln 3 = \ln 1.5
y=ln⁡1.5ln⁡3≈0.36907y = \frac{\ln 1.5}{\ln 3} \approx 0.36907
y≈0.369 (to 3 s.f.)y \approx 0.369 \text{ (to 3 s.f.)}

Answer

y=0y = 0 or y=0.369y = 0.369 (to 3 s.f.)

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