JEE Main · Mathematics ↓ Falling

Binomial Theorem appeared 37 times across 3 years — 4.3% of Mathematics. This question is from Sum of Binomial Coefficients.

Year 2026 2025 2024 Total
Questions 9 17 11 37

If ¹¹C₁2+ ¹¹C₂3+ + ¹¹C₉10=(n)/(m) with (n,m)=1, then n+m is equal to

Numerical Answer Type:
Enter a numerical value Answer: 2041 to 2041 +4 marks

Solution & Explanation

Related Formula
(ⁿCᵣ)/(r+1) = ⁿ⁺¹Cᵣ₊₁n+1 Σk=0ⁿ ⁿCk = 2ⁿ
Core Logic

The given series can be rewritten using summation notation:

S = Σr=1⁹ ¹¹Cᵣr+1

Applying the coefficient shifting identity (ⁿCᵣ)/(r+1) = ⁿ⁺¹Cᵣ₊₁n+1:

S = Σr=1⁹ ¹²Cᵣ₊₁12 S = (1)/(12) Σr=1⁹ ¹²Cᵣ₊₁
Step 1: Expand and Complete the Series

Expand the internal sum by shifting the index bounds:

S = (1)/(12) ( ¹²C₂ + ¹²C₃ + + ¹²C₁₀ )

We know the complete sum of binomial coefficients for n=12 is 2¹². We just need to subtract the missing boundary terms: k = 0, 1, 11, 12.

2¹² = Σk=0¹² ¹²Ck

The missing terms evaluate to: ¹²C₀ = 1 ¹²C₁ = 12

¹²C₁₁ = 12

¹²C₁₂ = 1 Sum of missing terms = 1 + 12 + 12 + 1 = 26.

Step 2: Calculate Final Fraction

Substitute this back into the series equation:

S = (1)/(12) [ 2¹² - 26 ] S = (1)/(12) [ 4096 - 26 ] S = (4070)/(12)

Simplify the fraction by dividing by 2 to achieve the coprime structure (n)/(m):

S = (2035)/(6)

Thus, n = 2035 and m = 6, and they are coprime ((2035, 6) = 1).

Calculate n + m:

n + m = 2035 + 6 = 2041
Pattern Recognition

Whenever you see binomial coefficients divided by their sequential index (r+1), always use the absorption identity (1)/(n+1) n+1r+1 to bump the top index up by 1. Then fill the array to force the complete 2ⁿ⁺¹ sum.

Chapter Mix

Class 11 Mathematics: Binomial Theorem

Reference Study Guides

More Binomial Theorem Previous-Year Questions — Page 8

Q2 jee_main_2024_31_jan_morning Sum of Coefficients and Limits
Let a be the sum of all coefficients in the expansion of (1 - 2x + 2x²)²⁰²³ (3 - 4x² + 2x³)²⁰²⁴ and b = x → 0 ( ∫₀x (1 + t)t²⁰²⁴ + 1 dtx² ). If the equations cx² + dx + e = 0 and 2bx² + ax + 4 = 0 have a common root, where c, d, e in R, then d : c : e equals
  • A. 2:1:4
  • B. 4:1:4
  • C. 1:2:4
  • D. 1:1:4

Solution

Core Logic

To find the sum of all coefficients in a polynomial expansion, substitute x = 1.

a = (1 - 2(1) + 2(1)²)²⁰²³ (3 - 4(1)² + 2(1)³)²⁰²⁴ a = (1)²⁰²³ (1)²⁰²⁴ = 1
Step 1: Evaluate Limit for b

Evaluate b = x → 0 ∫₀x ln(1 + t)1 + t²⁰²⁴ dtx² Using L'Hôpital's Rule (differentiating numerator via Newton-Leibniz):

b = x → 0 ln(1 + x)1 + x²⁰²⁴2x = x → 0 (ln(1 + x))/(x) × 12(1 + x²⁰²⁴) b = 1 × (1)/(2) = (1)/(2)
Step 2: Analyze Common Roots

The given second equation is 2bx² + ax + 4 = 0. Substitute a = 1 and b = (1)/(2):

2((1)/(2))x² + 1(x) + 4 = 0 x² + x + 4 = 0

The discriminant of x² + x + 4 = 0 is D = 1 - 16 < 0. Roots are non-real complex conjugates.

Step 3: Final Ratio

Since c, d, e in R and one root is common with a quadratic having non-real roots, both roots must be common. Thus, the coefficients must be proportional:

(c)/(1) = (d)/(1) = (e)/(4)

This implies d : c : e = 1 : 1 : 4.

Pattern Recognition

If a quadratic equation with real coefficients shares a common root with another quadratic having complex roots (D < 0), both roots must be shared, meaning their coefficients are directly proportional.

Chapter Mix

Class 11 Maths: Binomial Theorem Class 12 Maths: Limits and Derivatives Class 11 Maths: Quadratic Equations

Q25 jee_main_2024_31_jan_morning Coefficients in Expansion
In the expansion of (1 + x)(1 - x²)(1 + (3)/(x) + (3)/(x²) + (1)/(x³))⁵, x ≠ 0, the sum of the coefficient of x³ and x⁻¹³ is equal to
Numerical Answer. Answer: 118 to 118

Solution

Core Logic
(1+x)(1-x²) ( (1 + (1)/(x))³ )⁵ = (1+x)(1-x)(1+x) (x+1)¹⁵x¹⁵ = (1-x)(1+x)¹⁷x¹⁵ = (1+x)¹⁷ - x(1+x)¹⁷x¹⁵
Step 1: Find Coefficient of x^3

To find coeff of x³ in (1+x)¹⁷ - x(1+x)¹⁷x¹⁵, we need the coeff of x¹⁸ in the numerator (1+x)¹⁷ - x(1+x)¹⁷. The maximum power of x in (1+x)¹⁷ is 17, and in x(1+x)¹⁷ is 18. Coeff of x¹⁸ in (1+x)¹⁷ is 0. Coeff of x¹⁸ in x(1+x)¹⁷ is the coeff of x¹⁷ in (1+x)¹⁷, which is 1717 = 1. Thus, coeff of x¹⁸ in the numerator is 0 - 1 = -1.

Step 2: Find Coefficient of x^{-13}

To find coeff of x⁻¹³, we need the coeff of x² in the numerator (1+x)¹⁷ - x(1+x)¹⁷. Coeff of x² in (1+x)¹⁷ is 172. Coeff of x² in x(1+x)¹⁷ is coeff of x¹ in (1+x)¹⁷, which is 171. Value = 172 - 171 = (17 × 16)/(2) - 17 = 136 - 17 = 119.

Step 3: Final Sum

Sum of coefficients = -1 + 119 = 118.

Chapter Mix

Class 11 Maths: Binomial Theorem

More Binomial Theorem Questions — jee_main_2024_29_jan_morning

Practice all Binomial Theorem previous-year questions →

Rankbit System
JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%) | JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%)