Відповіді §.17 - §.18 » §.18 (2)

Знайдіть суму перших n членів арифметичної прогресії (xn), якщо: 1. n = 18, 2x1 + 17d = –11. S18 = (2x_1+ 17d)/2 • 18 = (-11)/2 • 18 = –99. 2. n = 10, x2 + x9 = 25, S10 – ?. S10 = (2x_1+ 9d)/2 • 10. x2 + x9 = x1 + d + x1 + 8d = 2x1 + 9d = 25. S10 = 25/2 • 10 = 125. 3. n = 24, x5 = –34, x20 = 92. x5 + x20 = –34 + 92 = 58. x5 + x20 = x1 + 4d + x1 + 19d = 2x1 + 23d = 58. S24 = (2x1+23d)/2 • 24 = 58/2 • 24 = 696. 4. n = 51, x26 = 100. x26 = x1 + 26d. S51 = (2x_1+ 50d)/2 • 51 = (x1 + 25d) • 51 = 100 • 51 = 5100.