Question

Помогите пожалуйста)))

Answer 1

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

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