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  1. (a)

    One of the roots of the equation 3x3+px2+qx+3=03x^3 + px^2 + qx + 3 = 0, where pp and qq are real, is 1+2i1 + \sqrt{2}\mathrm{i}. Find the other roots and the values of pp and qq.

  2. (b)

    The complex numbers ww and zz, with ∣w∣<∣z∣|w| < |z|, satisfy the simultaneous equations
    wz∗=3−iandw+z∗=2−i.wz^* = 3 - \mathrm{i} \quad \text{and} \quad w + z^* = 2 - \mathrm{i}.
    Find ww and zz.

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Worked solution

7(a)

1. Identify the second root using the conjugate root theorem.

Since the polynomial has real coefficients pp and qq, the complex roots must occur in conjugate pairs.
Therefore, the second root is 1−2i1 - \sqrt{2}\mathrm{i}.

2. Form the quadratic factor corresponding to the two complex roots.

The quadratic factor formed by the complex conjugate roots is:
(x−(1+2i))(x−(1−2i))(x - (1 + \sqrt{2}\mathrm{i}))(x - (1 - \sqrt{2}\mathrm{i}))
=((x−1)−2i)((x−1)+2i)= ((x - 1) - \sqrt{2}\mathrm{i})((x - 1) + \sqrt{2}\mathrm{i})
=(x−1)2−(2i)2= (x - 1)^2 - (\sqrt{2}\mathrm{i})^2
=(x2−2x+1)−(−2)= (x^2 - 2x + 1) - (-2)
=x2−2x+3= x^2 - 2x + 3

3. Determine the third root by introducing a linear factor and comparing constant terms.

Let the remaining linear factor be 3x+c3x + c. The cubic polynomial can be factored as:
3x3+px2+qx+3=(x2−2x+3)(3x+c)3x^3 + px^2 + qx + 3 = (x^2 - 2x + 3)(3x + c)
Comparing the constant terms on both sides:
3c=3  ⟹  c=13c = 3 \implies c = 1
The linear factor is 3x+13x + 1. Setting this to 00 gives the third root:
3x+1=0  ⟹  x=−133x + 1 = 0 \implies x = -\frac{1}{3}

4. Find the values of $p$ and $q$ by expanding the polynomial factors.

Substitute c=1c = 1 and expand the expression:
(x2−2x+3)(3x+1)=x2(3x+1)−2x(3x+1)+3(3x+1)(x^2 - 2x + 3)(3x + 1) = x^2(3x + 1) - 2x(3x + 1) + 3(3x + 1)
=3x3+x2−6x2−2x+9x+3= 3x^3 + x^2 - 6x^2 - 2x + 9x + 3
=3x3−5x2+7x+3= 3x^3 - 5x^2 + 7x + 3
Comparing this with 3x3+px2+qx+33x^3 + px^2 + qx + 3, we deduce the unknown coefficients:
p=−5p = -5
q=7q = 7

Answer

The other roots are 1−2i1 - \sqrt{2}\mathrm{i} and −13-\frac{1}{3}.
p=−5p = -5, q=7q = 7.

7(b)

1. Express $z^*$ in terms of $w$ and substitute into the first equation to form a quadratic equation.

From w+z∗=2−iw + z^* = 2 - \mathrm{i}, we have:
z∗=2−i−wz^* = 2 - \mathrm{i} - w

Substitute this into wz∗=3−iwz^* = 3 - \mathrm{i}:
w(2−i−w)=3−iw(2 - \mathrm{i} - w) = 3 - \mathrm{i}
2w−iw−w2=3−i2w - \mathrm{i}w - w^2 = 3 - \mathrm{i}
w2−(2−i)w+(3−i)=0w^2 - (2 - \mathrm{i})w + (3 - \mathrm{i}) = 0

2. Solve the quadratic equation to find the possible values of $w$.

Using the quadratic formula for w2−(2−i)w+(3−i)=0w^2 - (2 - \mathrm{i})w + (3 - \mathrm{i}) = 0:
w=(2−i)±(2−i)2−4(1)(3−i)2w = \frac{(2 - \mathrm{i}) \pm \sqrt{(2 - \mathrm{i})^2 - 4(1)(3 - \mathrm{i})}}{2}
w=2−i±4−4i+i2−12+4i2w = \frac{2 - \mathrm{i} \pm \sqrt{4 - 4\mathrm{i} + \mathrm{i}^2 - 12 + 4\mathrm{i}}}{2}
w=2−i±4−4i−1−12+4i2w = \frac{2 - \mathrm{i} \pm \sqrt{4 - 4\mathrm{i} - 1 - 12 + 4\mathrm{i}}}{2}
w=2−i±−92w = \frac{2 - \mathrm{i} \pm \sqrt{-9}}{2}
w=2−i±3i2w = \frac{2 - \mathrm{i} \pm 3\mathrm{i}}{2}

This gives two possible values for ww:
w1=2+2i2=1+iw_1 = \frac{2 + 2\mathrm{i}}{2} = 1 + \mathrm{i}
w2=2−4i2=1−2iw_2 = \frac{2 - 4\mathrm{i}}{2} = 1 - 2\mathrm{i}

3. Find the corresponding values of $z$ for each possible value of $w$.

Case 1: If w=1+iw = 1 + \mathrm{i}
z∗=2−i−(1+i)=1−2iz^* = 2 - \mathrm{i} - (1 + \mathrm{i}) = 1 - 2\mathrm{i}
Taking the conjugate yields z=1+2iz = 1 + 2\mathrm{i}.

Case 2: If w=1−2iw = 1 - 2\mathrm{i}
z∗=2−i−(1−2i)=1+iz^* = 2 - \mathrm{i} - (1 - 2\mathrm{i}) = 1 + \mathrm{i}
Taking the conjugate yields z=1−iz = 1 - \mathrm{i}.

4. Calculate the modulus of $w$ and $z$ in both cases to apply the condition $|w| < |z|$ and determine the final valid pair.

Check Case 1: w=1+i,z=1+2iw = 1 + \mathrm{i}, z = 1 + 2\mathrm{i}
∣w∣=12+12=2|w| = \sqrt{1^2 + 1^2} = \sqrt{2}
∣z∣=12+22=5|z| = \sqrt{1^2 + 2^2} = \sqrt{5}
Since 2<5\sqrt{2} < \sqrt{5}, this pair satisfies ∣w∣<∣z∣|w| < |z|.

Check Case 2: w=1−2i,z=1−iw = 1 - 2\mathrm{i}, z = 1 - \mathrm{i}
∣w∣=12+(−2)2=5|w| = \sqrt{1^2 + (-2)^2} = \sqrt{5}
∣z∣=12+(−1)2=2|z| = \sqrt{1^2 + (-1)^2} = \sqrt{2}
Since 5≮2\sqrt{5} \not< \sqrt{2}, this pair is rejected.

Hence, w=1+iw = 1 + \mathrm{i} and z=1+2iz = 1 + 2\mathrm{i}.

Answer

w=1+i,z=1+2iw = 1 + \mathrm{i}, z = 1 + 2\mathrm{i}

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