Question

вычислите 1*4+2*7+3*10+4*13+..+9*28

Answer 1

S9 = 1*4+2*7+3*10+4*13+..+9*28

an = n×(1 + 3n)

S9 = a1 + a2 + a3 + a4 + a5 + a6 + a7 + a8 + a9 = (1 × 4) + (2 × 7) + (3 × 10) + (4 × 13) + (5 × 16) + (6 × 19) + (7 × 22) + (8 × 25) + (9 × 28) = 4 + 14 + 30 + 52 + 80 + 114 + 154 + 200 + 252 = 100 + 80 + 366 + 154 + 200 = 100 + 80 + 520 + 200 = 300 + 600 = 900

Ответ: 900