Question 0521

Sigma Notation
2021 Paper 1 Question 5 Variant

Question

(a)
Express
5x+21(x+3)(x+4)(x+5)\frac{5 x + 21}{(x + 3)(x + 4)(x + 5)}
in partial fractions.
[3]
(b)
Hence find, in terms of n,{n,}
r=1n5r+21(r+3)(r+4)(r+5).\sum_{r=1}^n \frac{5 r + 21}{(r + 3)(r + 4)(r + 5)}.
[3]
(c)
State the value of
r=15r+21(r+3)(r+4)(r+5).\sum_{r=1}^\infty \frac{5 r + 21}{(r + 3)(r + 4)(r + 5)}.
[1]

Answer