Đáp án: $N = \frac{75}{46}$
Giải thích các bước giải:
$ N = \frac{5}{1.4} + \frac{5}{4.7} + \frac{5}{7.10} + ...+ \frac{5}{40.43} + \frac{5}{43.46}$
$ = \frac{5}{3}.(\frac{3}{1.4} + \frac{3}{4.7} + \frac{3}{7.10} + ...+ \frac{3}{40.43} + \frac{3}{43.46})$
$ = \frac{5}{3}.(\frac{1}{1} - \frac{1}{4} + \frac{1}{4} - \frac{1}{7}+ \frac{1}{7} - \frac{1}{10}+ ...+ \frac{1}{40} - \frac{1}{43}+ \frac{1}{43} - \frac{1}{46})$
$ = \frac{5}{3}.(1 - \frac{1}{46}) = \frac{5}{3}.(\frac{46 - 1}{46}) = \frac{75}{46}$
