AP Class 9 Mathematics
1. Real Numbers
Rational Numbers — Numbers that CAN be expressed as p/q (p,q ∈ Z, q≠0)
- Terminating decimals: Denominator has ONLY 2 and/or 5 as prime factors. Example: 3/8 = 0.375 (8 = 2³).
- Non-terminating RECURRING decimals: Denominator has prime factors OTHER than 2 and 5. Example: 1/3 = 0.333... (3 is not 2 or 5).
Irrational Numbers — CANNOT be expressed as p/q
- √2, √3, √5, π. Proof that √2 is irrational (by contradiction — assume p/q in simplest form → contradiction).
Real Numbers on the Number Line
'Every real number corresponds to EXACTLY one point on the number line — and every point corresponds to EXACTLY one real number.'
2. Polynomials
Key Definitions
| Term | Definition | Example |
|---|---|---|
| Degree | HIGHEST power of the variable | 3x⁴+2x²: degree = 4 |
| Zero of polynomial | Value of x for which p(x)=0 | p(2)=0 → 2 is a zero |
| Constant polynomial | Degree 0 | p(x)=5 |
| Zero polynomial | ALL coefficients = 0 | p(x)=0 (degree undefined) |
Remainder Theorem
When p(x) is divided by (x−a), the remainder = p(a). 'This is POWERFUL — you don't need to do long division. Just EVALUATE the polynomial at x=a.'
Factor Theorem
(x−a) is a factor of p(x) IF AND ONLY IF p(a) = 0. 'The converse of the Remainder Theorem. If p(a)=0 → (x−a) is a FACTOR.'
Algebraic Identities
- (a+b)² = a²+2ab+b²
- (a−b)² = a²−2ab+b²
- a²−b² = (a+b)(a−b)
- (x+a)(x+b) = x²+(a+b)x+ab
- (a+b+c)² = a²+b²+c²+2ab+2bc+2ca
- a³+b³ = (a+b)(a²−ab+b²)
- a³−b³ = (a−b)(a²+ab+b²)
Factorisation Methods
| Method | When to Use |
|---|---|
| Common Factor | All terms share a factor |
| Grouping | Group terms with common factors |
| Splitting Middle Term | For quadratics: find two numbers whose product = ac and sum = b |
3. Coordinate Geometry
The Cartesian Plane
- x-axis (horizontal). y-axis (vertical). Origin O(0,0).
- Quadrants: I (+,+). II (−,+). III (−,−). IV (+,−).
Plotting Points
A point (x,y) is plotted by: x units along the x-axis. y units parallel to the y-axis.
Key Formulas
- Distance from x-axis = |y|. Distance from y-axis = |x|.
4. Lines and Angles
Angle Types: Acute (<90°). Right (=90°). Obtuse (90°–180°). Straight (=180°). Reflex (180°–360°).
Angle Pairs
| Relationship | Property | Example |
|---|---|---|
| Complementary | Sum = 90° | 30°+60° |
| Supplementary | Sum = 180° | 110°+70° |
| Vertically Opposite | EQUAL | When two lines intersect |
| Adjacent | Share common arm and vertex | Neighbouring angles |
Parallel Lines and a Transversal
Corresponding angles = EQUAL. Alternate interior angles = EQUAL. Interior angles on the same side of transversal = SUPPLEMENTARY (sum = 180°).
5. Triangles
Types: By Sides (Equilateral, Isosceles, Scalene). By Angles (Acute, Right, Obtuse).
Congruence Criteria
SSS (Side-Side-Side). SAS (Side-Angle-Side). ASA (Angle-Side-Angle). AAS (Angle-Angle-Side). RHS (Right angle-Hypotenuse-Side).
Key Properties
- Angle sum = 180°. Exterior angle = sum of two interior OPPOSITE angles.
- Larger SIDE → opposite Larger ANGLE.
6. Quadrilaterals
| Shape | Key Properties |
|---|---|
| Parallelogram | Opposite sides ∥ & =. Diagonals BISECT each other. |
| Rhombus | All sides =. Diagonals ⟂. |
| Rectangle | All angles 90°. Diagonals =. |
| Square | All properties of rhombus AND rectangle. |
| Trapezium | ONE pair of ∥ sides. |
Angle Sum of Quadrilateral = 360°.
7. Surface Areas and Volumes
| Solid | Surface Area | Volume |
|---|---|---|
| Cube | 6s² | s³ |
| Cuboid | 2(lb+bh+hl) | lbh |
| Cylinder | 2πr(r+h) | πr²h |
| Cone | πr(r+l) [l=√(r²+h²)] | ⅓πr²h |
| Sphere | 4πr² | (4/3)πr³ |
| Hemisphere | 3πr² | (2/3)πr³ |
8. Statistics
Data Collection and Presentation
Primary data (collected directly). Secondary (from existing sources). Frequency distribution tables. Histograms (continuous data — bars TOUCH). Frequency polygons.
Measures of Central Tendency
- Mean: X̄ = Σx/n (ungrouped). X̄ = Σfx/Σf (grouped).
- Median: Middle value when ordered.
- Mode: Most frequent value.
Exam Strategy
| Unit | Approx. Marks |
|---|---|
| Number System & Algebra | 17-20 |
| Geometry (Lines, Triangles, Quadrilaterals) | 15-18 |
| Coordinate Geometry | 6-8 |
| Mensuration | 13-15 |
| Statistics | 8-10 |
Common Mistakes to Avoid
- Forgetting to mention rational numbers must have q≠0.
- Confusing Remainder Theorem (divide by x−a, remainder = p(a)) with Factor Theorem (p(a)=0 → x−a is a factor).
- Mixing SA and volume formulas for cone vs cylinder — the ⅓ factor for cone volume is commonly missed.
