Do not use a calculator in answering this question.
- (a)
One of the roots of the equation , where and are real, is . Find the other roots and the values of and .
- (b)
The complex numbers and , with , satisfy the simultaneous equations
Find and .
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Worked solution
7(a)
1. Identify the second root using the conjugate root theorem.
Since the polynomial has real coefficients and , the complex roots must occur in conjugate pairs.
Therefore, the second root is .
2. Form the quadratic factor corresponding to the two complex roots.
The quadratic factor formed by the complex conjugate roots is:
3. Determine the third root by introducing a linear factor and comparing constant terms.
Let the remaining linear factor be . The cubic polynomial can be factored as:
Comparing the constant terms on both sides:
The linear factor is . Setting this to gives the third root:
4. Find the values of $p$ and $q$ by expanding the polynomial factors.
Substitute and expand the expression:
Comparing this with , we deduce the unknown coefficients:
The other roots are and .
, .
7(b)
1. Express $z^*$ in terms of $w$ and substitute into the first equation to form a quadratic equation.
From , we have:
Substitute this into :
2. Solve the quadratic equation to find the possible values of $w$.
Using the quadratic formula for :
This gives two possible values for :
3. Find the corresponding values of $z$ for each possible value of $w$.
Case 1: If
Taking the conjugate yields .
Case 2: If
Taking the conjugate yields .
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:
Since , this pair satisfies .
Check Case 2:
Since , this pair is rejected.
Hence, and .
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