Progressions — Arithmetic and Geometric
Arithmetic Progression (AP)
A sequence where the DIFFERENCE between consecutive terms is CONSTANT (= d).
nth Term: aₙ = a + (n−1)d
Sum: Sₙ = n/2[2a + (n−1)d] = n/2(a + l)
Geometric Progression (GP)
A sequence where the RATIO between consecutive terms is CONSTANT (= r).
nth Term: aₙ = arⁿ⁻¹
Sum: Sₙ = a(rⁿ−1)/(r−1) [r>1] or Sₙ = a(1−rⁿ)/(1−r) [r<1]
Coordinate Geometry
Distance Formula: d = √[(x₂−x₁)² + (y₂−y₁)²]
Section Formula (Internal): ((mx₂+nx₁)/(m+n), (my₂+ny₁)/(m+n))
Midpoint: ((x₁+x₂)/2, (y₁+y₂)/2)
Area of Triangle: ½|x₁(y₂−y₃) + x₂(y₃−y₁) + x₃(y₁−y₂)|. Area = 0 → COLLINEAR.
Detailed Formulas for Arithmetic Progression
General Form of an AP
An arithmetic progression is represented as: a, a+d, a+2d, a+3d, ...
nth Term — Deep Dive
The nth term formula aₙ = a + (n−1)d gives the value of ANY term without listing all previous terms.
Example 1: Find the 10th term of the AP: 2, 5, 8, 11, ... Here a = 2, d = 3, n = 10. a₁₀ = 2 + (10−1)×3 = 2 + 27 = 29.
Example 2: The 5th term of an AP is 17 and the 10th term is 32. Find the AP. Given: a₅ = a + 4d = 17 ... (i) a₁₀ = a + 9d = 32 ... (ii) Subtract (i) from (ii): 5d = 15 → d = 3 Substitute in (i): a + 12 = 17 → a = 5 So the AP is: 5, 8, 11, 14, ... ✓
Sum of n Terms — Derivation
Sₙ = a + (a+d) + (a+2d) + ... + [a+(n−2)d] + [a+(n−1)d] Write in REVERSE order: Sₙ = [a+(n−1)d] + [a+(n−2)d] + ... + (a+d) + a Add the two equations. Each pair sums to 2a + (n−1)d. There are n pairs. 2Sₙ = n[2a + (n−1)d] Sₙ = n/2[2a + (n−1)d] ✓
When the LAST TERM l = a + (n−1)d is known: Sₙ = n/2(a + l)
Example 3: Find the sum of the first 20 terms of the AP: 1, 3, 5, 7, ... a = 1, d = 2, n = 20 S₂₀ = 20/2[2×1 + (20−1)×2] = 10[2 + 38] = 10 × 40 = 400 ✓
Example 4: How many terms of the AP: 24, 21, 18, ... must be taken for the sum to be 78? a = 24, d = −3, Sₙ = 78 78 = n/2[48 + (n−1)(−3)] 78 = n/2[48 − 3n + 3] 78 = n/2[51 − 3n] 156 = n(51 − 3n) 156 = 51n − 3n² 3n² − 51n + 156 = 0 n² − 17n + 52 = 0 (n − 4)(n − 13) = 0 n = 4 or n = 13 Both are valid! For n = 4, sum = 78. For n = 13, sum is ALSO 78 because later terms are NEGATIVE and cancel out. 'Understand this — sometimes more terms don't mean MORE sum.'
Applications of AP
- Simple Interest: Equal amounts added each year — forms an AP of interest amounts. 'If you deposit Rs. 1000 at 5% simple interest, you earn Rs. 50 EVERY year — it never changes.'
- Stacking: Bricks in a triangular stack — each row has a fixed number fewer than the row below.
- Depreciation: Value decreases by a CONSTANT amount each year.
Harmonic Progression (HP)
A sequence whose RECIPROCALS form an AP. General form: 1/a, 1/(a+d), 1/(a+2d), ...
Example: 1/2, 1/5, 1/8, 1/11, ... is an HP because 2, 5, 8, 11, ... is an AP.
There is NO direct formula for the sum of an HP. To solve HP problems, convert to AP, solve, then convert BACK.
Geometric Progression — Deep Dive
General Form of a GP
a, ar, ar², ar³, ...
Types of GP
| Type | Condition | Example |
|---|---|---|
| Finite GP | Limited number of terms | 1, 2, 4, 8 |
| Infinite GP | Unlimited terms | 1, 1/2, 1/4, 1/8, ... |
| Increasing GP | r > 1 | 2, 6, 18, 54, ... |
| Decreasing GP | 0 < r < 1 | 81, 27, 9, 3, ... |
| Constant GP | r = 1 | 5, 5, 5, 5, ... |
| Alternating GP | r < 0 | 1, −3, 9, −27, ... |
nth Term — Examples
Example 5: Find the 8th term of the GP: 3, 6, 12, 24, ... a = 3, r = 2, n = 8 a₈ = 3 × 2⁷ = 3 × 128 = 384 ✓
Example 6: The 3rd term of a GP is 12 and the 6th term is 96. Find the GP. a₃ = ar² = 12 ... (i) a₆ = ar⁵ = 96 ... (ii) Divide (ii) by (i): r³ = 96/12 = 8 → r = 2 Substitute in (i): a × 4 = 12 → a = 3 The GP is: 3, 6, 12, 24, 48, 96, ... ✓
Sum of n Terms — Derivation
Sₙ = a + ar + ar² + ... + arⁿ⁻¹ Multiply by r: rSₙ = ar + ar² + ar³ + ... + arⁿ Subtract: Sₙ − rSₙ = a − arⁿ Sₙ(1 − r) = a(1 − rⁿ) Sₙ = a(1 − rⁿ)/(1 − r) [for r ≠ 1]
For r > 1, it is convenient to write: Sₙ = a(rⁿ − 1)/(r − 1)
Example 7: Find the sum of the first 6 terms of the GP: 2, 6, 18, ... a = 2, r = 3, n = 6 S₆ = 2(3⁶ − 1)/(3 − 1) = 2(729 − 1)/2 = 728 ✓
Sum of Infinite GP
When |r| < 1, as n → ∞, rⁿ → 0 S∞ = a/(1 − r) [for |r| < 1]
Example 8: Find the sum of the infinite GP: 1, 1/2, 1/4, 1/8, ... a = 1, r = 1/2 S∞ = 1/(1 − 1/2) = 1/(1/2) = 2 ✓
Applications of GP
- Compound Interest: The amount at the end of each year forms a GP. 'Compound interest is GP in action — your money grows by a RATIO, not a constant amount.'
- Population growth: If population grows at 2% per year, it forms a GP.
- Radioactive decay: The amount remaining halves every half-life — a GP with r = 1/2.
- Installment payments: Many loan repayment schedules follow GP patterns.
Common Mistakes — AP and GP
| Mistake | Correction |
|---|---|
| Confusing AP and GP formulas | 'AP uses ADDITION (d). GP uses MULTIPLICATION (r). Remember: A for ADDITION.' |
| Using r = 1 formula without checking | Direct formula gives division by zero. Treat r = 1 as Sₙ = n×a |
| Forgetting n−1 in nth term | aₙ = a + (n−1)d, NOT a + nd |
| Wrong ratio in GP | r = a₂/a₁, always check with a₃/a₂ too |
| Using AP sum for GP sum | Never mix Sₙ formulas |
AP SSC Board Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| nth term of AP/GP | 2-3 | Very High |
| Sum of n terms of AP | 3-4 | Very High |
| Sum of n terms of GP | 3-4 | High |
| Application problems | 4-5 | Moderate |
| Finding a and d/r from conditions | 4 | High |
| Infinite GP sum | 2 | Moderate |
Self-Test Questions
- Find the 15th term of the AP: 7, 11, 15, 19, ...
- The 7th term of an AP is 34 and the 13th term is 64. Find the AP.
- Find the sum of the first 25 terms of the AP: 3, 7, 11, 15, ...
- How many terms of the AP: 63, 60, 57, ... give a sum of 693?
- Find the 10th term of the GP: 2, 6, 18, 54, ...
- The 2nd term of a GP is 6 and the 5th term is 162. Find the GP.
- Find the sum of the first 8 terms of the GP: 1, 3, 9, 27, ...
- Find the sum of the infinite GP: 4, 2, 1, 1/2, ...
- A man saves Rs. 1250 in the first year, Rs. 1350 in the second year, Rs. 1450 in the third year, and so on. How much does he save in 15 years?
- A bacteria culture doubles every hour. If there are 100 bacteria now, how many will there be after 8 hours?
Answers: 1) 63, 2) AP: 4, 9, 14, ..., 3) 1275, 4) 14 terms, 5) 39366, 6) GP: 2, 6, 18, 54, ..., 7) 3280, 8) 8, 9) Rs. 28,500, 10) 25,600
