Question

Умнейшие люди, помогите, пожалуйста.
3 и 4 пункты... ​

Answer 1

Пошаговое объяснение:

$sn = \frac{2a1 + (n - 1) \times d}{2} \times n = \frac{a1 + an}{2} \times n$

3.

$72.5 = \frac{0.5 + 14}{2} \times n$

$n = \frac{725}{10} \times \frac{2 \times 10}{145} = \frac{2 \times 5}{1} = 10$

$an = a1 + d(n - 1)$

$14 = 0.5 + (10 - 1) \times d$

$14 - 0.5 = 9d$

$d = \frac{135}{10} \times \frac{1}{9} = \frac{27}{2 \times 9} = \frac{3}{2} = 1.5$

4.

$17.5 = \frac{a1 + 16}{2} \times 7$

$35 = 7(a1 + 16)$

$5 = a1 + 16$

$a1 = - 11$

$an = a1 + d(n - 1)$

$16 = - 11 +d (7 - 1)$

$27 = 6d$

$d = \frac{27}{6} = \frac{9}{2} = 4.5$