Do not use a calculator in answering this question. The equation 2z3+5z2+pz+q=02z^3 + 5z^2 + pz + q = 0 has a root z=−1+iz = -1 + \mathrm{i}, where pp and qq are real constants.
Find pp and qq and all the other roots of the equation.

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Worked solution

3

1. Identify the complex conjugate root using the property of real coefficients.

Since the coefficients pp and qq are real constants, the complex conjugate of the given root must also be a root of the equation.
Therefore, z=−1−iz = -1 - \mathrm{i} is another root.

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

The quadratic factor having these roots is:
(z−(−1+i))(z−(−1−i))(z - (-1 + \mathrm{i}))(z - (-1 - \mathrm{i}))
=((z+1)−i)((z+1)+i)= ((z + 1) - \mathrm{i})((z + 1) + \mathrm{i})
=(z+1)2−i2= (z + 1)^2 - \mathrm{i}^2
=z2+2z+1−(−1)= z^2 + 2z + 1 - (-1)
=z2+2z+2= z^2 + 2z + 2

3. Set up an identity to find the remaining linear factor.

Since 2z3+5z2+pz+q=02z^3 + 5z^2 + pz + q = 0 is a cubic equation with leading coefficient 22, it can be expressed as the product of the quadratic factor and a linear factor (2z+c)(2z + c):
2z3+5z2+pz+q=(z2+2z+2)(2z+c)2z^3 + 5z^2 + pz + q = (z^2 + 2z + 2)(2z + c)
Expanding the right side gives:
=2z3+cz2+4z2+2cz+4z+2c= 2z^3 + cz^2 + 4z^2 + 2cz + 4z + 2c
=2z3+(c+4)z2+(2c+4)z+2c= 2z^3 + (c + 4)z^2 + (2c + 4)z + 2c
Comparing the coefficient of z2z^2:
5=c+4  ⟹  c=15 = c + 4 \implies c = 1

4. Compare the remaining coefficients to solve for $p$ and $q$.

Substitute c=1c = 1 into the expanded expression and compare coefficients.
Comparing the coefficient of zz:
p=2c+4p = 2c + 4
p=2(1)+4=6p = 2(1) + 4 = 6
Comparing the constant term:
q=2cq = 2c
q=2(1)=2q = 2(1) = 2

5. Solve the linear factor to find the third root.

The linear factor is 2z+12z + 1. Setting this to zero gives the final root:
2z+1=02z + 1 = 0
z=−12z = -\frac{1}{2}
The other roots of the equation are therefore −1−i-1 - \mathrm{i} and −12-\frac{1}{2}.

Answer

p=6,q=2p = 6, q = 2
Other roots: −1−i-1 - \mathrm{i} and −12-\frac{1}{2}

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