Find the complex number z in the form x+iy, where x, y \in \mathbb{R} such that 3izz∗−1+2i=−i\dfrac{3i z}{z^{*}-1+2i} = -i. [Note: z∗z^{*} is the conjugate of z.] The two roots z1,z2z_{1}, z_{2} of the equation z2+z+p=0z^{2}+z+p=0, where p∈Rp \in \mathbb{R}, are such that z1=a+ibz_{1}=a+ib where a, b \in \mathbb{R}, b \ne 0andand|z_{1}-z_{2}|=\sqrt{3}$. Find the value of p.

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Worked solution

4(a)

1. Let $z = x+iy$ and substitute it into the given equation.

Let z=x+iyz = x+iy, which means the complex conjugate is z∗=x−iyz^{*} = x-iy. Substituting these into the given equation yields:

3i(x+iy)x−iy−1+2i=−i\frac{3i(x+iy)}{x-iy-1+2i} = -i

2. Multiply by the denominator and expand both sides.

Multiply both sides by (x−iy−1+2i)(x-iy-1+2i) to remove the fraction:

3i(x+iy)=−i(x−iy−1+2i)3i(x+iy) = -i(x - iy - 1 + 2i)

Expand the brackets:

3ix+3i2y=−ix+i2y+i−2i23ix + 3i^2y = -ix + i^2y + i - 2i^2

Substitute i2=−1i^2 = -1:

−3y+3ix=−ix−y+i+2-3y + 3ix = -ix - y + i + 2

Group the real and imaginary components together:

−3y+i(3x)=(2−y)+i(1−x)-3y + i(3x) = (2 - y) + i(1 - x)

3. Equate the real and imaginary parts to solve for $x$ and $y$.

Equating the real parts:
−3y=2−y-3y = 2 - y
−2y=2  ⟹  y=−1-2y = 2 \implies y = -1

Equating the imaginary parts:
3x=1−x3x = 1 - x
4x=1  ⟹  x=144x = 1 \implies x = \frac{1}{4}

Therefore, the complex number is z=14−iz = \frac{1}{4} - i.

Answer

z=14−iz = \frac{1}{4} - i

4(b)

1. Identify the relationship between the two complex roots.

Since the quadratic equation z2+z+p=0z^2+z+p=0 has real coefficients (1,1,p∈R1, 1, p \in \mathbb{R}) and a complex root z1=a+ibz_1 = a+ib (with b≠0b \ne 0), the roots must occur in conjugate pairs.
Therefore, z2=z1∗=a−ibz_2 = z_1^* = a - ib.

2. Use the difference of the roots to find $b^2$.

Substitute z1z_1 and z2z_2 into the given condition ∣z1−z2∣=3|z_1 - z_2| = \sqrt{3}:
∣(a+ib)−(a−ib)∣=3|(a+ib) - (a-ib)| = \sqrt{3}
∣2ib∣=3|2ib| = \sqrt{3}
2∣b∣=3  ⟹  ∣b∣=322|b| = \sqrt{3} \implies |b| = \frac{\sqrt{3}}{2}
Squaring both sides gives:
b2=34b^2 = \frac{3}{4}

3. Use the sum of roots to find $a$.

Using the relationship for the sum of roots (α+β=−ba\alpha + \beta = -\frac{b}{a}):
z1+z2=−1z_1 + z_2 = -1
(a+ib)+(a−ib)=−1(a+ib) + (a-ib) = -1
2a=−1  ⟹  a=−122a = -1 \implies a = -\frac{1}{2}

4. Use the product of roots to find $p$.

Using the relationship for the product of roots (αβ=ca\alpha\beta = \frac{c}{a}):
z1z2=pz_1 z_2 = p
(a+ib)(a−ib)=p(a+ib)(a-ib) = p
a2+b2=pa^2 + b^2 = p
Substitute a=−12a = -\frac{1}{2} and b2=34b^2 = \frac{3}{4}:
p=(−12)2+34p = \left(-\frac{1}{2}\right)^2 + \frac{3}{4}
p=14+34=1p = \frac{1}{4} + \frac{3}{4} = 1

Answer

p=1p = 1

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