Question

Помогите пожалуйста)))

Answer 1

$a_{5} = 10 \\ a_{10} = 25 \\ \\ a_{1} + 4d= 10 \\ a_{1} + 9d= 25 \\ 5d = 15 \\ d = 3 \\ a_{1} = 10 - 4d \\ a_{1} = 10 - 4 \times 3 = 10 - 12 = - 2$

$S_{n} = \frac{2 a_{1} + d(n - 1) }{2} \times n\\ S_{30} = \frac{2 a_{1} + 29d }{2} \times 30 \\ S_{30} = \frac{2 \times ( - 2)+ 29 \times 3}{2} \times 30 = \\ = ( - 4 + 87) \times 15 = 83 \times 15 = \\ = 1245$