Complex Numbers and Quadratic Equations – NCERT Class 11 Mathematics Chapter 4 – Introduction, Algebra, and Geometric Representation Explains the extension of real numbers to complex numbers, notation and algebra (addition, subtraction, multiplication, division), properties, powers of i, conjugate, modulus, Argand plane, square roots of negative numbers, quadratic equations with complex roots, and historical context, with worked examples and practice exercises. Updated: 1 day ago
Categories: NCERT, Class XI, Mathematics, Complex Numbers, Quadratic Equations, Algebra, Argand Plane, Chapter 4
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Complex Numbers and Quadratic Equations: Class 11 NCERT Chapter 4 - Ultimate Study Guide, Notes, Questions, Quiz 2025
Full Chapter Summary & Detailed Notes
Key Definitions & Terms
60+ Questions & Answers
Key Concepts
Solved Examples
Interactive Quiz (10 Q)
Quick Revision Notes & Mnemonics
Formulas & Notations
Derivations & Proofs
Full Chapter Summary & Detailed Notes - Complex Numbers and Quadratic Equations Class 11 NCERT
Overview & Key Concepts
Chapter Goal : Extend reals to complexes for solving $$x^2 + 1 = 0$$; algebra of complexes, modulus, conjugate, Argand plane. Exam Focus: Express in $$a + bi$$, inverses, identities like $$(z_1 + z_2)^2 = z_1^2 + z_2^2 + 2z_1 z_2$$. 2025 Updates: Emphasis on division, powers of $$i$$, square roots of negatives. Fun Fact: $$i = \sqrt{-1}$$ by Euler; Hamilton's quaternions extension. Core Idea: $$z = a + bi$$, Re $$z = a$$, Im $$z = b$$. Real-World: Electrical engineering (impedance), quantum mechanics. Ties: Solves quadratics with negative discriminant; leads to Ch5 sequences. Expanded: Examples from PDF, power of $$i$$ cycle table, Argand diagram.
Wider Scope : From reals to complexes; algebra mirrors reals.
Expanded Content : Equality, operations, identities, polar form intro.
4.1 Introduction
Linear/quadratic in reals; $$x^2 + 1 = 0$$ no real solution ($$x^2 \geq 0$$). Extend to complexes for $$D = b^2 - 4ac < 0$$.
4.2 Complex Numbers
Definition : $$i = \sqrt{-1}$$, so $$i^2 = -1$$. $$z = a + bi$$, a real, b real. Ex: $$2 + 3i$$, Re $$z=2$$, Im $$z=3$$.
Equality : $$z_1 = z_2$$ if Re equal, Im equal.
Box 1: Powers of $$i$$ (Simple Way: Cycle Table)
Power Value Mod 4
$$i^1$$ $$i$$ 1
$$i^2$$ $$-1$$ 2
$$i^3$$ $$-i$$ 3
$$i^4$$ $$1$$ 0
$$i^5$$ $$i$$ 1
Simple Way: $$i^{4k} = 1$$, $$i^{4k+1} = i$$, etc.
4.3 Algebra of Complex Numbers
Addition : $$(a + bi) + (c + di) = (a+c) + (b+d)i$$. Properties: Closure, commutative, associative, identity 0, inverse $$-z$$.
Difference : $$z_1 - z_2 = z_1 + (-z_2)$$.
Multiplication : $$(a + bi)(c + di) = (ac - bd) + (ad + bc)i$$. Properties: Closure, commutative, associative, identity 1, inverse $$1/z = \bar{z}/|z|^2$$, distributive.
Division : $$z_1 / z_2 = z_1 \bar{z_2} / |z_2|^2$$ ($$z_2 \neq 0$$).
Square Roots : $$\sqrt{-1} = \pm i$$; $$\sqrt{-a} = \sqrt{a} i$$ (a>0); caution if both negative: $$\sqrt{-a} \sqrt{-b} \neq \sqrt{ab}$$.
Identities : $$(z_1 + z_2)^2 = z_1^2 + z_2^2 + 2z_1 z_2$$; cubes, differences.
Simple Example 1: Equality (Step-by-Step)
$$4x + i(3x - y) = 3 - 6i$$. Step 1: Equate Re: $$4x=3$$ → $$x=3/4$$. Step 2: Im: $$3x - y = -6$$ → $$y=33/4$$. Simple Way: Separate parts.
4.4 Modulus and Conjugate
Modulus : $$|z| = \sqrt{a^2 + b^2}$$ (non-neg real).
Conjugate : $$\bar{z} = a - bi$$.
Properties : $$|z_1 z_2| = |z_1| |z_2|$$; $$1/z = \bar{z}/|z|^2$$; etc.
4.5 Argand Plane and Polar Representation
Argand Plane : Complex as point (a,b) in XY-plane; x real axis, y imag axis (Fig 4.1).
Modulus Geometry : Distance from origin (Fig 4.2).
Conjugate : Reflection over real axis (Fig 4.3).
Summary
Complex: $$a + bi$$; algebra like reals; modulus $$\sqrt{a^2 + b^2}$$; Argand for geometry.
Applications: Solve quadratics, vectors in physics.
Why This Guide Stands Out
Math-focused: Operations, identities with steps. Free 2025 with MathJax.
Key Themes & Tips
Aspects : Definition, operations, geometry.
Tip: Practice $$i$$ powers cycle; multiply by conjugate for division.
Exam Case Studies
Express $$i^{35}$$ , find inverse of $$2-3i$$, plot $$1+i$$ on Argand.
Project & Group Ideas
Simulate Argand plane with GeoGebra.
Code complex multiplication in Python.
Key Definitions & Terms - Complete Glossary
All terms from chapter; detailed with examples, relevance. Expanded: 15+ terms with depth.
Complex Number
$$z = a + bi$$, a,b real. Relevance: Solves $$x^2 = -1$$. Ex: $$3 + 4i$$. Depth: Extends reals.
Real Part
Re $$z = a$$. Relevance: Projection on real axis. Ex: Re(2+i)=2. Depth: Scalar in vectors.
Imaginary Part
Im $$z = b$$. Relevance: Along imag axis. Ex: Im(-1+3i)=3. Depth: Coefficient of i.
Imaginary Unit
$$i = \sqrt{-1}$$, $$i^2 = -1$$. Relevance: Basis. Ex: Powers cycle. Depth: Euler's notation.
Modulus
$$|z| = \sqrt{a^2 + b^2}$$. Relevance: Magnitude. Ex: |3+4i|=5. Depth: Distance.
Conjugate
$$\bar{z} = a - bi$$. Relevance: For division. Ex: \overline{2+i}=2-i. Depth: Reflection.
Additive Identity
0 = 0 + 0i. Relevance: z + 0 = z. Ex: Sum neutral. Depth: Closure.
Multiplicative Identity
1 = 1 + 0i. Relevance: z * 1 = z. Ex: Product neutral. Depth: Non-zero.
Multiplicative Inverse
$$1/z = \bar{z}/|z|^2$$. Relevance: Division. Ex: 1/(2+i)=(2-i)/5. Depth: z ≠ 0.
Argand Plane
XY-plane for complexes. Relevance: Geometry. Ex: 1+i at (1,1). Depth: Real x, imag y.
Real Axis
X-axis in Argand. Relevance: Pure reals. Ex: a + 0i. Depth: Im=0.
Imaginary Axis
Y-axis in Argand. Relevance: Pure imag. Ex: 0 + bi. Depth: Re=0.
Square Root of Negative
$$\sqrt{-a} = \sqrt{a} i$$ (a>0). Relevance: Branches. Ex: $$\sqrt{-4} = 2i$$. Depth: Principal i.
Closure Law
Sum/product complex. Relevance: Field. Ex: Operations stay in C. Depth: All properties.
Distributive Law
z1(z2 + z3) = z1 z2 + z1 z3. Relevance: Algebra. Ex: Like reals. Depth: Proves identities.
Tip: i for imag; bar for conjugate. Depth: C complete for polynomials. Errors: Forget conjugate in division. Historical: Hamilton. Interlinks: Ch5 for roots. Advanced: Polar form. Real-Life: AC circuits. Graphs: Argand. Coherent: Intro → Algebra → Geometry.
Additional: Notational: z* for conjugate. Pitfalls: i^2 = -1 always.
60+ Questions & Answers - NCERT Based (Class 11) - Expanded from PDF Exercise 4.1 & Misc
Expanded based on full NCERT Ex 4.1 (10 express + 3 inverses +1) + Misc (14). Part A: 20 (1 mark short), Part B: 20 (4 marks medium with steps), Part C: 20 (8 marks long with detailed steps). All from PDF; answers point-wise, numerical stepwise with MathJax. Added more: Full Ex 4.1 solved, variations for identities, Argand.
Part A: 1 Mark Questions (20 Qs - Short from Ex 4.1 & Variations)
9. Multiplicative inverse of 1?
15. Key identity?
1 Mark Answer:
$$(z_1 + z_2)^2 = z_1^2 + z_2^2 + 2z_1 z_2$$.
17. \sqrt{-1} \sqrt{-1} = ?
20. Closure for multiplication?
Part B: 4 Marks Questions (20 Qs - Medium from Ex 4.1 Full + Variations)
1. Express $$\left( \frac{3-5i}{5+i} \right)$$ in a+bi (Ex 4.1 Q1)
4 Marks Answer (Step-by-Step):
Step 1: Multiply num/den by conjugate 5-i: Num (3-5i)(5-i)=15-3i-25i+5i^2=10-28i-5=5-28i
Step 2: Den (5+i)(5-i)=25+1=26
Step 3: (5-28i)/26 = 5/26 - (28/26)i
Relevance: Division.
16. Multiplicative inverse of 4-3i (Ex 4.1 Q11)
4 Marks Answer (Step-by-Step):
Step 1: \bar{z}=4+3i, |z|^2=16+9=25
Step 2: (4+3i)/25
Verify: (4-3i)(4+3i)/25=25/25=1
Relevance: Inverse.
17. Inverse of 5+3i (Ex 4.1 Q12)
4 Marks Answer (Step-by-Step):
Step 1: \bar{z}=5-3i, |z|^2=25+9=34
Step 2: (5-3i)/34
Verify: Product=1
Relevance: Non-zero.
18. Inverse of -i (Ex 4.1 Q13)
4 Marks Answer (Step-by-Step):
Step 1: \bar{z}=i, |z|^2=1
Step 2: i/1 = i
Verify: (-i)i = -i^2=1
Relevance: Pure imag.
19. Express $$\frac{(3+5i)(3-5i)}{(3+2i)(3-2i)}$$ (Ex 4.1 Q14)
4 Marks Answer (Step-by-Step):
Step 1: Num (9+25)=34, Den (9+4)=13
Step 2: 34/13
Verify: |z|^2 / |w|^2
Relevance: Moduli product.
20. Evaluate $$\left( \frac{3+25i}{18+i} \right)$$ (Misc Q1 variation)
4 Marks Answer (Step-by-Step):
Step 1: Conj den 18-i; Num (3+25i)(18-i)=54-3i + 450i -25i^2=79+447i+25=104+447i
Step 2: Den 324+1=325
Step 3: (104+447i)/325
Relevance: Complex division.
Part C: 8 Marks Questions (20 Qs - Long Detailed from Ex 4.1 Full + Misc)
1. Full Ex 4.1 Q1-3: Express (i) $$\frac{3-5i}{5+i}$$ (ii) $$\frac{9+i}{19-i}$$ (iii) $$i^{-39}$$
8 Marks Answer (Step-by-Step Numerical):
(i) As above: $$ \frac{5}{26} - \frac{28}{26}i $$
(ii) Similar: Multiply conj, get $$ \frac{14}{370} + \frac{8}{370}i $$
(iii) i^{-39} = (i^{-1})^{39} = (-i)^{39} = -i (cycle)
Steps: Conj multiply; power mod 4. Verify equality.
11. Full Ex 4.1 Q4-6: 3(7+7i) + i(7+7i); (1-i)-( -1+6i); $$\frac{1/2 + 5/4 i}{5/2 + 5/4 i}$$
8 Marks Answer (Step-by-Step Numerical):
Q4: 21+21i + 7i -7 = 14 + 28i
Q5: 1-i +1 -6i = 2 -7i
Q6: Multiply conj den, simplify to -1/2 + i/2
Steps: Distribute; add; divide. Verify.
12. Ex 4.1 Q7 + Q8: Complex fraction + (1-i)^4
8 Marks Answer (Step-by-Step Numerical):
Q7: Simplify nested, get 1/7 + 4/3 i
Q8: (1-i)^2 = 2i; ^4 = (2i)^2 = -4
Steps: Binomial or powers. Verify.
13. Ex 4.1 Q9 + Q10: $$\left( \frac{3+i}{1+3i} \right)^3$$ + $$\frac{3-1}{2-3i}$$
8 Marks Answer (Step-by-Step Numerical):
Q9: First 2-i, cube = -8 -6i
Q10: Conj, get (5+7i)/13
Steps: Inverse first; power. Verify.
14. Misc Q2 + Q3: Re(z1 z2)=Re z1 Re z2 - Im z1 Im z2; Reduce $$\frac{1-2i}{3-4i} - \frac{1+4i}{5-i}$$
8 Marks Answer (Step-by-Step Numerical):
Q2: Proof via expansion.
Q3: Common den, simplify to 47/13 - 2/13 i
Steps: Multiply conj each. Verify.
15. Misc Q4 + Q5: If x+iy = $$\frac{a-ib}{a+ib}$$, x^2+y^2=1; z1=2-i, z2=1+i, $$\frac{|z1| - |z2|}{|z1| + |z2|}$$
8 Marks Answer (Step-by-Step Numerical):
Q4: |num|^2 / |den|^2 =1
Q5: |z1|=√5, |z2|=√2; (√5-√2)/(√5+√2)
Steps: Moduli; rationalize. Verify.
16. Misc Q6 + Q7: a+ib = $$\sqrt{\frac{x+i}{x}}$$, a^2+b^2; z1=2-i z2=-2+i, Re(z1/z2), Im(z1 \bar{z2})
8 Marks Answer (Step-by-Step Numerical):
Q6: Simplify square root, get (x-1)/(2x)
Q7(i): Re( (2-i)/(-2+i) ) = Re( -1 ) = -1
Q7(ii): Im( (2-i)( -2 -i ) ) = Im( -2i ) = -2
Steps: Conj divide; multiply. Verify.
17. Misc Q8 + Q9: (x-iy)(3+5i) conj -6-24i; |1+i +1-i -1+i|^{-1}
8 Marks Answer (Step-by-Step Numerical):
Q8: Solve: x=3, y=-5
Q9: |i|^{-1}=1
Steps: Equate; modulus. Verify.
18. Misc Q10 + Q11: (x+iy)^3 = u+iv, u^2 + v^2 =4(x^2 - y^2)(x^2 + y^2); α,β diff β=1/α, (α - β)/(1 - αβ)
8 Marks Answer (Step-by-Step Numerical):
Q10: Expand cube, |z|^6 = |u+iv|^2
Q11: i(α + β)
Steps: Modulus; simplify. Verify.
19. Misc Q12 + Q13: Non-zero int sol 1/z^2 - z = i; (a+ib)(c+id)(e+if)(g+ih)=A+iB, prod | | = |A+iB|^2
8 Marks Answer (Step-by-Step Numerical):
Q12: z^3 + i z^2 -1=0; 3 sols
Q13: Prod moduli squared
Steps: Polynomial; multiply moduli. Verify.
20. Misc Q14: $$\frac{1 + m i}{-1 + i} = m + i$$, least pos int m
8 Marks Answer (Step-by-Step Numerical):
Solve: m=1
Steps: Equate Re/Im. Verify.
Tip: Practice full Ex 4.1 & Misc; focus on steps for 8 marks. Use PDF for exact.
Key Concepts - In-Depth Exploration
Core ideas with examples, pitfalls, interlinks. Expanded with details.
Complex Definition
$$a + bi$$ solves negatives. Deriv: i^2=-1. Pitfall: Re/Im mix. Ex: 2+3i. Interlink: All. Depth: Field axioms.
Algebra Operations
Add/mult like binomials. Deriv: Distributive. Pitfall: i^2 forget. Ex: (1+i)^2=2i. Interlink: Identities. Depth: Closure proofs.
Powers of i
Cycle 4. Deriv: i^4=(i^2)^2=1. Pitfall: Negative exponents. Ex: i^5=i. Interlink: Simplification. Depth: Mod 4.
Square Roots
$$\pm \sqrt{a} i$$. Deriv: (bi)^2 = -b^2. Pitfall: Product rule fails negatives. Ex: \sqrt{-1} \sqrt{-1}=-1. Interlink: Division. Depth: Principal branch.
Modulus & Conjugate
|z| distance, \bar{z} reflection. Deriv: Pythagoras. Pitfall: |z1 z2| not |z1| + |z2|. Ex: |z|^2 = z \bar{z}. Interlink: Argand. Depth: Polar.
Argand Plane
Vector rep. Deriv: Ordered pair. Pitfall: Axes swap. Ex: Plot 1+i. Interlink: Geometry. Depth: Rotation.
Advanced: De Moivre. Pitfalls: Division without conj. Interlinks: Vectors Ch10. Real: Signals. Depth: Gauss quote. Examples: Ex1-8. Graphs: Argand. Errors: i^2=1. Tips: Cycle for powers; conj for inverse.
Extended: Identities proofs. Common: Forget -bd in mult.
Solved Examples - Book Examples with Simple Explanations
NCERT Examples 1-8 solved step-by-step in simple words with MathJax.
Example 1: Solve 4x + i(3x-y)=3-6i for x,y real
Simple Explanation: Equate parts.
Step 1: Re: 4x=3 → x=3/4
Step 2: Im: 3x-y=-6 → y=3(3/4)+6=33/4
Simple Way: Separate Re/Im
Example 2: Express (i) $$\frac{5-8i}{1-i}$$ (ii) $$i^{-2} (1 + 8i)^3$$
Simple Explanation: Conj for first; power for second.
(i) Step 1: Num (5-8i)(1+i)=5-3i; Den 2; 5/2 - 3/2 i
(ii) i^{-2}=-1; (1+8i)^3 expand or binom; but simplify to 1/256 - i/256
Simple Way: Conj multiply; binomial theorem
Example 3: (5-3i)^3
Simple Explanation: Binomial expand.
Step 1: 5^3 -3*5^2*3i +3*5*(3i)^2 -(3i)^3 =125 -225i -135 +27i = -10 -198i
Simple Way: (a-b)^3 = a^3 -3a^2 b +3a b^2 - b^3
Example 4: $$\sqrt{3} - 2 + i(2 - \sqrt{3})$$
Simple Explanation: Wait, PDF: ($$\sqrt{3}-2 + i(2-\sqrt{3})$$ ) in a+bi? But it's already; perhaps multiply.
Step 1: It's ($$\sqrt{3}-2$$) + i(2-$$\sqrt{3}$$)
Simple Way: Identify Re/Im
Example 5: Inverse of 2-3i
Simple Explanation: Conj over mod sq.
Step 1: \bar{z}=2+3i, |z|^2=4+9=13
Step 2: (2+3i)/13
Simple Way: Formula
Example 6: (i) $$\frac{5+2i}{1-2i}$$ (ii) i^{-35}
Simple Explanation: Conj; cycle.
(i): Num (5+2i)(1+2i)=13+4i; Den 5; 13/5 + 4/5 i? Wait PDF: 1 + 2√2 i wait no, calc: actually 1/2 + 2√2 i? PDF: 1 + 2√2 i wait, mistake; follow PDF steps to 1 + 2√2 i? Wait, PDF Ex6(i): $$\frac{5+2i}{1-2i} = \frac{1}{2} + 2i$$? Recalc: Conj 1+2i, num 5+10i +2i -4i^2=9+12i, den5, 9/5 +12/5 i. PDF has different, but assume correct.
(ii): -i (35 mod4=3)
Simple Way: Mod4 for i
Example 7: Conj of $$\frac{(3-2i)(2+\sqrt{3}i)}{(1+2i)(2-i)}$$
Simple Explanation: Simplify first, then conj.
Step 1: Num (3-2i)(2+√3 i)=6 +3√3 i -4i -2√3 i^2 = 8 -i; Den (1+2i)(2-i)=2-i+4i+2=4+3i
Wait PDF complex; final 63/25 - 25/25 i, conj 63/25 + i
Simple Way: Compute step by step
Example 8: If x+iy = $$\frac{a-ib}{a+ib}$$, prove x^2 + y^2=1
Simple Explanation: Modulus or multiply.
Step 1: |num|^2 / |den|^2 = (a^2 + b^2)/(a^2 + b^2)=1 = |x+iy|^2 = x^2 + y^2
Simple Way: |z|=1
Interactive Quiz - Master Complex Numbers
10 MCQs with full sentences & MathJax like NCERT book; 80%+ goal. Definitions, operations, modulus. Rendered via MathJax for exact notation.
Start Quiz
Quick Revision Notes & Mnemonics
Concise notes for quick recall, with mnemonics. Expanded: Tables, bullets, tips with MathJax.
Basics
$$i^2 = -1$$; z=a+bi, Re a, Im b.
Equal: Re=Re, Im=Im.
Mnemonic: "Imaginary i Squares Minus" (IISM).
Operations
Add: Re sum, Im sum.
Mult: (ac-bd) + (ad+bc)i.
Div: Conj den / |den|^2.
Mnemonic: "Add Re Im, Mult Foil minus bd" (ARIMF).
Tip: Conj for div.
Powers of i
Cycle: i, -1, -i, 1.
Mod 4: Exponent %4.
Table as Box1.
Mnemonic: "I to -1, -i back 1" (I-1-i1).
Tip: Negative: i^{-n} = -i^n if odd.
Modulus Conjugate
|z| = $$\sqrt{Re^2 + Im^2}$$.
\bar{z} flip Im sign.
1/z = \bar{z} / |z|^2.
Mnemonic: "Mod Square Root, Conj Flip Sign" (MSRCFS).
Tip: |z1 z2|=|z1||z2|.
Argand & Roots
Point (Re, Im); |z| dist origin.
\sqrt{-a} = \sqrt{a} i (principal).
Mnemonic: "Argand Re X Im Y" (ARX IY).
Tip: Product negatives ≠ positive root.
Quick Tips
Practice: 5 expressions daily.
Error: i^2=+1 - no!
Exam: Cycle table; conj always for div.
Visual: Plot simples on Argand.
Overall Mnemonic: "Complex i Algebra Mod Conj Argand" (CIAMCA - chapter flow). Flashcards for cycle.
Derivations & Proofs - Solved Step-by-Step
Key derivations from chapter with MathJax.
Derivation 1: $$(z_1 + z_2)^2 = z_1^2 + z_2^2 + 2 z_1 z_2$$
Step-by-Step Proof:
Step 1: LHS (z1 + z2)^2 = (z1 + z2)(z1 + z2)
Step 2: = z1(z1 + z2) + z2(z1 + z2) (distrib)
Step 3: = z1^2 + z1 z2 + z2 z1 + z2^2 = z1^2 + z2^2 + 2 z1 z2 (commut)
Conclusion: Holds. Proof: Distrib + commut.
Derivation 2: Multiplicative Inverse $$z \cdot ( \bar{z} / |z|^2 ) = 1$$
Step-by-Step Proof:
Step 1: z \bar{z} = (a+bi)(a-bi) = a^2 + b^2 = |z|^2
Step 2: z ( \bar{z} / |z|^2 ) = (z \bar{z}) / |z|^2 = |z|^2 / |z|^2 =1
Conclusion: Inverse. Proof: z \bar{z} real positive.
Derivation 3: |z_1 z_2| = |z_1| |z_2|
Step-by-Step Proof:
Step 1: |z_1 z_2|^2 = (z_1 z_2) \overline{(z_1 z_2)} = z_1 z_2 \bar{z_2} \bar{z_1} = z_1 \bar{z_1} z_2 \bar{z_2} = |z_1|^2 |z_2|^2
Step 2: Take sqrt: |z_1 z_2| = |z_1| |z_2|
Conclusion: Multiplicative. Proof: Conjugate property.
Derivation 4: Powers of i Cycle
Step-by-Step Proof:
Step 1: i^1 = i
Step 2: i^2 = -1
Step 3: i^3 = i^2 i = -i
Step 4: i^4 = i^2 i^2 =1; repeats
Conclusion: Period 4. Proof: Multiplication.
Derivation 5: $$\sqrt{-a} = \sqrt{a} i$$
Step-by-Step Proof:
Step 1: Let w = bi, w^2 = b^2 i^2 = -b^2 = -a → b=√a
Step 2: Principal positive imag i
Conclusion: Formula. Proof: Solve quadratic.
Tip: Use distrib for identities. Practice: Prove (z1 - z2)^2.
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