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%)

Let the coefficient of x^r in the expansion of (x+3)^n-1 + (x+3)^n-2(x+2) + (x+3)^n-3(x+2)^2 + dots + (x+2)^n-1 be alpha_r. If sum_r=0^n alpha_r = beta^n - gamma^n, where beta, gamma in mathbbN, then the value of beta^2 + gamma^2 equals

Numerical Answer Type:
Enter a numerical value Answer: 25 to 25 +4 marks

Solution & Explanation

### Related Formula textSum of coefficients in a polynomial P(x) text is found by putting x=1. textSum of G.P.: S_n = fraca(r^n - 1)r - 1 ### Core Logic Let the expanded polynomial be P(x). The sum of its coefficients is sum alpha_r = P(1). Substitute x = 1 into the given expression: P(1) = 4^n-1 + 4^n-2(3) + 4^n-3(3^2) + dots + 3^n-1 This is a Geometric Progression with first term a = 4^n-1 and common ratio r = 3/4. There are n terms. P(1) = 4^n-1 frac1 - (3/4)^n1 - 3/4 = 4^n-1 frac1 - (3/4)^n1/4 = 4^n left(1 - frac3^n4^nright) = 4^n - 3^n Comparing this with beta^n - gamma^n, we get: beta = 4, quad gamma = 3 Calculate beta^2 + gamma^2: beta^2 + gamma^2 = 4^2 + 3^2 = 16 + 9 = 25 ### Pattern Recognition Substituting x=1 immediately bypasses expanding individual x^r terms for questions asking for sum of coefficients. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Maths: Binomial Theorem Class 11 Maths: Sequences and Series

Reference Study Guides

More Binomial Theorem Previous-Year Questions — Page 6

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 + 2x^2)^2023 (3 - 4x^2 + 2x^3)^2024 and b = lim_x to 0 left( fracint_0^x fraclog(1 + t)t^2024 + 1 dtx^2 right). If the equations cx^2 + dx + e = 0 and 2bx^2 + ax + 4 = 0 have a common root, where c, d, e in mathbbR, 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 = lim_x to 0 fracint_0^x fracln(1 + t)1 + t^2024 dtx^2 Using L'Hôpital's Rule (differentiating numerator via Newton-Leibniz): b = lim_x to 0 fracfracln(1 + x)1 + x^20242x = lim_x to 0 fracln(1 + x)x times frac12(1 + x^2024) b = 1 times frac12 = frac12 ### Step 2: Analyze Common Roots The given second equation is 2bx^2 + ax + 4 = 0. Substitute a = 1 and b = frac12: 2left(frac12right)x^2 + 1(x) + 4 = 0 implies x^2 + x + 4 = 0 The discriminant of x^2 + x + 4 = 0 is D = 1 - 16 < 0. Roots are non-real complex conjugates. ### Step 3: Final Ratio Since c, d, e in mathbbR and one root is common with a quadratic having non-real roots, both roots must be common. Thus, the coefficients must be proportional: fracc1 = fracd1 = frace4 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. ### Evaluation Rubric / Model Answer null ### 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 - x^2)left(1 + frac3x + frac3x^2 + frac1x^3right)^5, x neq 0, the sum of the coefficient of x^3 and x^-13 is equal to
Numerical Answer. Answer: 118 to 118

Solution

### Core Logic (1+x)(1-x^2) left( left(1 + frac1xright)^3 right)^5 = (1+x)(1-x)(1+x) frac(x+1)^15x^15 = frac(1-x)(1+x)^17x^15 = frac(1+x)^17 - x(1+x)^17x^15 ### Step 1: Find Coefficient of x^3 To find coeff of x^3 in frac(1+x)^17 - x(1+x)^17x^15, we need the coeff of x^18 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 x^18 in (1+x)^17 is 0. Coeff of x^18 in x(1+x)^17 is the coeff of x^17 in (1+x)^17, which is binom1717 = 1. Thus, coeff of x^18 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 x^2 in the numerator (1+x)^17 - x(1+x)^17. Coeff of x^2 in (1+x)^17 is binom172. Coeff of x^2 in x(1+x)^17 is coeff of x^1 in (1+x)^17, which is binom171. Value = binom172 - binom171 = frac17 times 162 - 17 = 136 - 17 = 119. ### Step 3: Final Sum Sum of coefficients = -1 + 119 = 118. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 11 Maths: Binomial Theorem

More Binomial Theorem Questions — jee_main_2024_31_jan_evening

Practice all Binomial Theorem previous-year questions →

YOUR FIRST PREP STEP STARTS HERE

We Map Every Repeating Question in Competitive Exams.

Say goodbye to generic mock test fatigue. RankBit uses smart analysis to group past exam questions into their foundational Repeating Question Types. Find chapter weightage, track repeating questions, and score higher with targeted practice.

Select Your Target Exam

Choose an exam track below to find formulas per chapter and patterns.

Syncing Exam Intelligence

Mapping formulas and patterns across all tracks…

PATH A — FULL LENGTH PRACTICE

Full Mock Test Hub

Simulate real NTA exam conditions with fully tracked mocks. Time yourself against past papers.

Under Development
PATH B — TARGETED PRACTICE

Topic-wise Practice Hub

Practice past-year questions one chapter at a time. Pick an exam → subject → chapter and get every PYQ for that topic — pulled together from all past papers — with the chapter's key formulas alongside.

Loading Questions... Browse Topics
Latest from the Blog
View all →

Loading articles...