A sequence u1,u2,u3,… u_1, u_2, u_3, \dots\,u1,u2,u3,… is defined by
un=20×0.85nu_n = 20 \times 0.85^nun=20×0.85n
Use an algebraic method to find the least value of N N\,N for which
∑n=1∞un−∑n=1Nun<1\sum_{n=1}^{\infty} u_n - \sum_{n=1}^{N} u_n < 1∑n=1∞un−∑n=1Nun<1
308 exam-style questions on AQA A Level Maths 1.7 D: Sequences and series, covering 1.7.1 Binomial expansion, 1.7.2 Types of sequence (A-level only), 1.7.3 Sigma notation (A-level only), 1.7.4 Arithmetic sequences and series (A-level only), 1.7.5 Geometric sequences and series (A-level only), and 1.7.6 Sequences and series in modelling (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.