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Worked solution

Solve coupled equations involving a conjugate

The complex numbers w,zw,z satisfy wz=32i\dfrac wz=\dfrac32i and z∗−2w=4+4iz^*-2w=4+4i. Find ww and zz. marks

Review: [2013/RVHS/I/7]; Lecture Example 9: [2014/RVHS/Prelim/I/7(part)] (the printed sources disagree).

CJC · Review problems · Question 3

Answer

z=−2+2iz=-2+2i, w=−3−3iw=-3-3i.

Substitute the ratio relation

The ratio requires z≠0z\ne0 and gives w=(3/2)izw=(3/2)iz. Substitution yields z∗−3iz=4+4iz^*-3iz=4+4i. Let z=x+iyz=x+iy; then x−iy−3i(x+iy)=x+3y+(−y−3x)ix-iy-3i(x+iy)=x+3y+(-y-3x)i.

How did this part go?

Solve for the real coordinates

Compare parts: x+3y=4x+3y=4 and −y−3x=4-y-3x=4. Multiply the first by 3 to get 3x+9y=123x+9y=12; adding the second gives 8y=168y=16, hence y=2y=2. Substitute into x+3y=4x+3y=4: x=−2x=-2. Thus z=−2+2iz=-2+2i.

How did this part go?

Find w and verify

w=(3/2)i(−2+2i)=−3−3iw=(3/2)i(-2+2i)=-3-3i. Check w/z=(3/2)iw/z=(3/2)i and z∗−2w=(−2−2i)−2(−3−3i)=4+4iz^*-2w=(-2-2i)-2(-3-3i)=4+4i. The source’s next line after y=2y=2 is a substitution giving x=−2x=-2; a duplicated “8y=16” in the extraction is a text-extraction artifact, not an extra printed equation.

How did this part go?

Common mistakes

Conjugation changes the sign of the imaginary part only. Keep the i² contribution when expanding −3i(x+iy).

Solution notes

Source filename says 2026, but its printed header says 2025/JC2; both are retained without silently changing the source identity.

This question also appears as Lecture Example 9. Both sources give the same equations and solution. Their printed exam references disagree (2013 in the review; 2014 in the lecture); both are retained without asserting which attribution is correct.

Read the hint