[Cambridge/H2 Maths 2007] The complex number ww is such that ww∗+2w=3+4iww^* + 2w = 3 + 4i, where w∗w^* is the complex conjugate of ww. Find ww in the form a+iba + ib, where aa and bb are real.

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Worked solution

Q2

1. Substitute $w=a+ib$ into the given equation to express it in terms of $a$ and $b$.

Let w=a+ibw = a + ib, then the complex conjugate is w∗=a−ibw^* = a - ib.
ww∗=(a+ib)(a−ib)=a2+b2ww^* = (a+ib)(a-ib) = a^2 + b^2
Substitute into ww∗+2w=3+4iww^* + 2w = 3 + 4i:
a2+b2+2(a+ib)=3+4ia^2 + b^2 + 2(a + ib) = 3 + 4i
a2+b2+2a+2bi=3+4ia^2 + b^2 + 2a + 2bi = 3 + 4i

2. Equate the real and imaginary parts to form simultaneous equations.

By comparing the real and imaginary parts on both sides:
Imaginary part:
2b=4  ⟹  b=22b = 4 \implies b = 2

Real part:
a2+b2+2a=3a^2 + b^2 + 2a = 3

3. Solve for $a$ by substituting the value of $b$.

Substitute b=2b=2 into the real part equation:
a2+(2)2+2a=3a^2 + (2)^2 + 2a = 3
a2+2a+4=3a^2 + 2a + 4 = 3
a2+2a+1=0a^2 + 2a + 1 = 0
(a+1)2=0  ⟹  a=−1(a + 1)^2 = 0 \implies a = -1

4. State the final complex number $w$.

Therefore, the complex number is:
w=−1+2iw = -1 + 2i

Answer

w=−1+2iw = -1 + 2i

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