Question

арефметичесеая прогрессии хелп​
найти an и Sn

Answer 1

Ответ:

$1)- 5. - 9......n = 9 \\ a1 = - 5 \\ a2 = - 9 \\ d = - 9 - ( - 5) = - 9 + 5 = - 4 \\ a9 = a1 + 8d = - 5 + 8 \times ( - 4) = \\ - 5 - 32 = - 37 \\ s9 = \frac{a1 + a9}{2} \times 9 \\ s9 = \frac{ - 5 + ( - 37)}{2} \times 9 = \frac{ - 42}{2} \times 9 = \\ = - 21 \times 9 = - 189$

$2)- 10.0......n = 15 \\ d = 0 - ( - 10) = 10 \\ a10 = a1 + 9d = - 10 + 9 \times 10 = 80 \\ s10 = \frac{ - 10 + 80}{2} \times 10 = 350$

$3)13.19.....n = </strong><strong>6</strong><strong> \\ d = 19 - 13 =6 \\ </strong><strong>a6</strong><strong> = 13 + </strong><strong>5</strong><strong> \times 6 = </strong><strong>43</strong><strong> \\ </strong><strong>s6</strong><strong> = \frac{13 + </strong><strong>43</strong><strong>}{2} \times </strong><strong>6</strong><strong>= </strong><strong>1</strong><strong>6</strong><strong>8</strong><strong>$