Use standard results from the list of formulae (MF19) to show that Σ (from r=1 to N) r(r+1)(3r+4) = (1/4)N(N+1)(N+2)(9N+19).
- (a)Use standard results from the list of formulae (MF19) to show that Σ (from r=1 to N) r(r+1)(3r+4) = (1/4)N(N+1)(N+2)(9N+19).[3]
- (b)Express (3r+4)/(r(r+1)) in partial fractions and hence use the method of differences to find Σ (from r=1 to N) (3r+4)/(r(r+1)) * (1/4)ʳ⁺¹ in terms of N.[4]
- (c)Deduce the value of Σ (from r=1 to infinity) (3r+4)/(r(r+1)) * (1/4)ʳ⁺¹.[1]
Mark scheme
Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.
- [B1] Expands.
- [M1] Substitutes formulae from MF19.
- [A1] AG
- [M1 A1] Finds partial fractions.
- [M1] Writes at least three terms, including last.
- [A1] (1/4) - (1/(4ⁿ⁺¹(N+1)))
- [B1] 1/4 CAO
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