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