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
231 exam-style questions on WJEC A Level Maths 3.4 Sequences and Series (A-level only), covering 3.4.1 Sequences and Series (A-level only), 3.4.2 Sequences and Series (A-level only), 3.4.3 Sequences and Series (A-level only), 3.4.4 Sequences and Series (A-level only), 3.4.5 Sequences and Series (A-level only), 3.4.6 Sequences and Series (A-level only), and 3.4.7 Sequences and Series (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.