Solution
Related Formula
(ⁿCᵣ)/(r+1) = ⁿ⁺¹Cᵣ₊₁n+1 Σk=0ⁿ ⁿCk = 2ⁿCore Logic
The given series can be rewritten using summation notation:
S = Σr=1⁹ ¹¹Cᵣr+1Applying the coefficient shifting identity (ⁿCᵣ)/(r+1) = ⁿ⁺¹Cᵣ₊₁n+1:
S = Σr=1⁹ ¹²Cᵣ₊₁12 S = (1)/(12) Σr=1⁹ ¹²Cᵣ₊₁Step 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 ¹²CkThe missing terms evaluate to: ¹²C₀ = 1 ¹²C₁ = 12
¹²C₁₁ = 12¹²C₁₂ = 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 = 2041Pattern 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