Complex numbers is one of the most reliable topics on the Philippine engineering board exam. It appears in Engineering Mathematics for every board — ECE, EE, ME, CE, ChE, and more. It connects directly to AC circuit analysis, phasor representation, impedance, Laplace transforms, and Fourier analysis. Master this topic and you are not just answering one section of the board exam. You are building the mathematical foundation that supports half of your engineering subjects.
This page is the complete hub for the PinoyBIX Complex Numbers ECE and EE Board Exam Reviewer Series. It contains a consolidated formula reference sheet covering all four parts of the series, a 25-item multiple choice practice exam drawn from all topics, and a full navigation guide to every post in the series. If you are short on time, start with the formula sheet. If you want to test yourself before the board exam, go straight to the practice exam.
- ECE (Electronics Engineer) — High frequency. Appears in Engineering Mathematics and Electronics subjects. All four topics in this series are tested. Expect 8 to 15 items total across the board exam.
- EE (Electrical Engineer) — Very high frequency. Complex numbers underpin the entire Electrical Circuits subject through impedance and phasor analysis. All four topics tested. Critical subject area.
- ME (Mechanical Engineer) — Moderate frequency. Engineering Mathematics covers forms, operations, and De Moivre’s theorem. AC circuit applications appear in Electrical Technology.
- CE (Civil Engineer) — Moderate frequency. Engineering Mathematics covers forms, operations, and basic De Moivre’s theorem. Power and impedance appear occasionally.
- ChE (Chemical Engineer) — Moderate frequency. Complex numbers appear in Engineering Mathematics and in process control through transfer functions and the Laplace variable
.
- GeE (Geodetic Engineer) — Low to moderate frequency. Basic forms and operations are tested in Engineering Mathematics.
- MetE and MinE — Low frequency. Basic forms, powers of
, and operations appear in Engineering Mathematics.
- Naval Architect and Marine Engineer — Moderate frequency. Impedance, phasors, and vibration analysis all use complex number mathematics.
The Complete Series — Four Parts
Each part of this series builds on the previous one. If you are new to complex numbers, read the parts in order. If you are reviewing a specific topic for the board exam, jump directly to the part you need.
| Part | Topic | Key Concepts | Problems | Best For |
|---|---|---|---|---|
| Part 1 | Forms and the j-Operator | Four forms, Argand diagram, modulus, argument, powers of |
10 | All boards — foundational, read this first |
| Part 2 | Operations | Addition, subtraction, multiplication, division, conjugate method, form selection rules | 10 | All boards — core computation |
| Part 3 | De Moivre’s Theorem | Integer powers, all nth roots, root spacing, principal root, root circle diagram | 10 | ECE, EE, ME, CE, ChE |
| Part 4 | AC Circuits and Applications | Impedance, phasors, Ohm’s law, series and parallel circuits, complex power, power factor | 10 | ECE, EE — critical topic |
How to Use This Series
If you have two weeks before the board exam, work through all four parts in order over the first week. Take the 25-item practice exam at the bottom of this page at the end of week one. Check your score against the answer key. For every item you got wrong, go back to the relevant part of the series, read the worked example that covers that problem type, and solve a similar problem from scratch before moving on.
If you have three days or less, go directly to the formula sheet below. Screenshot it or save it. Then take the practice exam and check your answers. Focus your remaining review time only on the topics where you got items wrong.
| Day | Activity | Time |
|---|---|---|
| Day 1 | Read Part 1 — Forms and the j-Operator. Work all 10 problems without looking at solutions first. Write down the powers of |
90 minutes |
| Day 2 | Read Part 2 — Operations. Work all 10 problems. Before moving on, make sure you can state the form selection rule from memory — rectangular for addition and subtraction, polar for multiplication and division. | 90 minutes |
| Day 3 | Read Part 3 — De Moivre’s Theorem. Work all 10 problems. Pay close attention to the nth root formula — the most common exam trap is writing only the principal root and missing the rest. | 90 minutes |
| Day 4 | Read Part 4 — AC Circuits and Applications. If you are ECE or EE, this is your most important day. Work all 10 problems and drill Ohm’s law in phasor form until it is automatic. | 2 hours |
| Day 5 | Take the 25-item practice exam at the bottom of this page. No notes. No formula sheet. Set a 40-minute timer and treat it like the real thing. Write your answers on paper before you scroll down to the key. | 40 minutes |
| Day 6 | Score your exam. For every item you got wrong, read the full solution in the Complete Solutions Post. If you missed three or more in any one part, go back to that part’s post — not just the solutions page — and work through similar problems from scratch. | 60 minutes |
| Day 7 | Review the formula sheet on this page from memory, not by reading it. Solve 5 additional problems of your own choosing without any reference. If you can do that cleanly, you are ready. | 45 minutes |
Quick Reference Formula Sheet
This consolidated formula sheet covers every key formula from all four parts of the series. Screenshot this section and keep it accessible during your review. On exam day, every formula on this sheet should come from memory — not from the screenshot.
Standard rectangular form:
Polar form:
Trigonometric form:
Exponential form and Euler’s formula:
Polar to rectangular:
Powers of — cycle of four:
Shortcut: Divide exponent by 4. Remainder , remainder
, remainder
, remainder
.
Imaginary unit definition:
Form selection rule:
- Addition and subtraction → use rectangular form
- Multiplication and division → use polar form
Addition and subtraction:
Multiplication in polar form:
Division in polar form:
Complex conjugate:
Division using conjugate method:
Power formula:
nth root formula:
Root spacing:
Key facts:
- Every complex number has exactly
distinct nth roots.
- All roots share the same modulus
.
- The principal root is
at
.
- Roots lie equally spaced on a circle of radius
.
Impedance:
Component impedances:
Series RLC impedance:
Parallel impedance (two elements):
Phasor conversion:
Ohm’s law in phasor form:
Complex power:
ELI the ICE man:
- ELI — In an inductor (L), voltage (E) leads current (I). Lagging power factor.
- ICE — In a capacitor (C), current (I) leads voltage (E). Leading power factor.
25-Item Practice Exam — All Topics Combined
This practice exam covers all four parts of the Complex Numbers series. Work through all 25 items without notes and without a reference sheet. Time yourself — allow 40 minutes for the full exam, which matches the approximate pace of the actual board exam. Check your answers against the key at the bottom of this page. For full solutions to every item, see the Complete Solutions Post.
- Time allowed: 40 minutes
- Number of items: 25
- Each item has one correct answer from four choices.
- No partial credit — each item is all or nothing.
- Passing score suggestion: 20 out of 25 (80%)
- Do not look at the answer key until you finish all 25 items.
Part A — Forms and the j-Operator (Items 1 to 5)
Item 1. What is the value of ?
(A) (B)
(C)
(D)
Item 2. Express in rectangular form.
(A) (B)
(C)
(D)
Item 3. Find the modulus and argument of .
(A) ,
(B)
,
(C)
,
(D)
,
Item 4. Which of the following is the exponential form of ?
(A) (B)
(C)
(D)
Item 5. Evaluate .
(A) (B)
(C)
(D)
Part B — Operations (Items 6 to 11)
Item 6. Compute .
(A) (B)
(C)
(D)
Item 7. Find .
(A) (B)
(C)
(D)
Item 8. Divide .
(A) (B)
(C)
(D)
Item 9. Simplify and express in rectangular form.
(A) (B)
(C)
(D)
Item 10. If , find
.
(A) (B)
(C)
(D)
Item 11. Multiply .
(A) (B)
(C)
(D)
Part C — De Moivre’s Theorem (Items 12 to 17)
Item 12. Evaluate .
(A) (B)
(C)
(D)
Item 13. Find the principal square root of .
(A) (B)
(C)
(D)
Item 14. How many distinct cube roots does any nonzero complex number have?
(A) (B)
(C)
(D)
Item 15. The angular spacing between consecutive fourth roots of any complex number is:
(A) (B)
(C)
(D)
Item 16. Find all cube roots of . Which of the following is NOT one of the cube roots?
(A) (B)
(C)
(D)
Item 17. Evaluate .
(A) (B)
(C)
(D)
Part D — AC Circuits and Applications (Items 18 to 25)
Item 18. A series circuit has and
. What is the magnitude of the total impedance?
(A) (B)
(C)
(D)
Item 19. A series RLC circuit has ,
, and
. Find the phase angle
.
(A) lagging (B)
leading (C)
leading (D)
lagging
Item 20. A voltage V is applied across an impedance
. Find the current
.
(A) A (B)
A (C)
A (D)
A
Item 21. The time domain current A is represented in phasor form as:
(A) (B)
(C)
(D)
Item 22. A load has complex power VA. What is the power factor?
(A) lagging (B)
lagging (C)
leading (D)
leading
Item 23. Two impedances and
are connected in parallel. Find
.
(A) (B)
(C)
(D)
Item 24. A single phase motor draws W at
power factor lagging from a
V rms source. What is the reactive power
?
(A) VAR (B)
VAR (C)
VAR (D)
VAR
Item 25 — Combined Topics. A complex number is raised to the fourth power to give
. The result
is then used as the impedance
in an AC circuit with source voltage
V. What is the magnitude of the phasor current
?
(A) A (B)
A (C)
A (D)
A
Answer Key
Full step-by-step solutions for every item are in the Complete Solutions Post — all 25 items in Given, Find, Solution format with examiner notes on each one.
| Item | Answer | Topic | Quick Explanation |
|---|---|---|---|
| 1 | D | Forms / j-operator | |
| 2 | A | Forms / conversion | |
| 3 | A | Forms / modulus and argument | |
| 4 | A | Forms / exponential | |
| 5 | A | Forms / j-operator | |
| 6 | A | Operations / addition | |
| 7 | C | Operations / multiplication | Magnitudes: |
| 8 | B | Operations / division | Magnitudes: |
| 9 | A | Operations / conjugate method | Multiply by |
| 10 | B | Operations / conjugate | |
| 11 | A | Operations / multiplication | |
| 12 | B | De Moivre’s / powers | |
| 13 | A | De Moivre’s / roots | |
| 14 | C | De Moivre’s / roots | Every nonzero complex number has exactly |
| 15 | C | De Moivre’s / root spacing | Spacing |
| 16 | D | De Moivre’s / roots | Cube roots of |
| 17 | A | De Moivre’s / powers | |
| 18 | C | AC Circuits / impedance | |
| 19 | A | AC Circuits / phase angle | |
| 20 | A | AC Circuits / Ohm’s law | |
| 21 | B | AC Circuits / phasors | Drop |
| 22 | B | AC Circuits / power factor | |
| 23 | D | AC Circuits / parallel impedance | |
| 24 | A | AC Circuits / complex power | |
| 25 | A | Combined — De Moivre’s + AC Circuits |
Score Interpretation
Match your score out of 25 against the bands below to see where you stand and what to do next.
| Score | Percentage | Reading | What to Do |
|---|---|---|---|
| 23 to 25 | 92% to 100% | Board Exam Ready | Check your missed items. If they cluster in one part, read that post once more. Otherwise move on to the next topic. |
| 18 to 22 | 72% to 88% | Passing Level | Find which parts you missed the most. Go back to those posts and work through the problems again from scratch, no solutions in front of you. |
| 13 to 17 | 52% to 68% | Needs More Work | Reread all four parts from the beginning. Work every problem without peeking. Take this exam again in three days. |
| Below 13 | Below 52% | Start Over | Go back to Part 1 and read it completely before touching any problems. The foundation is not there yet. That is fine — this series fixes that if you work through it properly. |
Frequently Asked Questions
Q1. Which part of this series should I study first if I have only one day before the board exam?
Start with the formula sheet on this page. Screenshot it and study it for 30 minutes. Then go to Part 1 and read only the Board Exam Quick Tips section and the powers of shortcut. Then read the Quick Tips from Part 2 focusing on the form selection rule. If you are taking the ECE or EE board exam, also read the Quick Tips from Part 4. With one day remaining, targeted review of high-yield rules is more effective than reading full post content from beginning to end.
Q2. Is De Moivre’s theorem really tested on the board exam or is it a minor topic?
It is tested. ECE board exams from multiple years have included problems asking for all cube roots or fourth roots of a complex number. The most common trap is writing only the principal root and missing the other roots. See Part 3 for the complete nth root formula and worked examples of the most common question types.
Q3. Do I need to memorize Euler’s formula or will it be given during the exam?
Memorize it. is not consistently provided in the reference data during Philippine engineering board exams. It is treated as required knowledge in the same category as the quadratic formula or the Pythagorean theorem. It appears in exponential form conversions, phasor derivations, Laplace transform problems, and Fourier series questions.
Q4. How is complex number mathematics connected to AC circuit analysis?
Impedance is a complex number in rectangular form. The impedance triangle is the Argand diagram. The modulus formula
is the Pythagorean theorem. The phase angle
is the argument formula. Ohm’s law in phasor form
is complex multiplication. Every computation you learned in Parts 1 through 3 reappears in AC circuit problems. This is why Part 4 is the most important post in the series for ECE and EE examinees.
Q5. What is the next series after Complex Numbers on PinoyBIX?
The next series covers Matrices and Determinants — another high-frequency topic in Engineering Mathematics for all engineering boards. It follows the same structure: four posts covering theory, operations, applications, and a combined practice exam with full solutions. Follow the PinoyBIX Facebook page to get notified when it goes live.
What is Next
Everything you need to pass the complex numbers section of any Philippine engineering board exam is in this series. The practice exam above gives you your score. The Complete Solutions Post gives you the step-by-step answer to every one of the 25 items. The four series posts give you the detailed worked examples when you need to rebuild a specific skill from the ground up.
The next series is Matrices and Determinants — the following topic in the PinoyBIX Engineering Mathematics sequence. It follows the same structure: five parts, worked problems, a combined practice exam, and full solutions. Follow PinoyBIX on Facebook to get notified when it goes live.
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