Question

A=13 d=-2 знайти а9 s18​

Answer 1

а9=а1+8d

a9=13+8*(-2)

a9=-3

S18=(a1+a18)*n/2

a18=a1+17d=13+17*(-2)=-21

S18=-8*18/2=-72

Answer 2

To find the 9th term (A9) and the 18th term (S18) of an arithmetic sequence with the first term A = 13 and a common difference of d = -2, you can use the following formulas:

1. Finding the nth term of an arithmetic sequence (A_n): A_n = A + (n - 1) * d

2. Finding the sum of the first n terms of an arithmetic sequence (S_n): S_n = n/2 * [2A + (n - 1) * d]

For A9:

A9 = 13 + (9 - 1) * (-2) A9 = 13 + 8 * (-2) A9 = 13 - 16 A9 = -3

So, A9 = -3.

For S18:

S18 = 18/2 * [2 * 13 + (18 - 1) * (-2)] S18 = 9 * [26 + 17 * (-2)] S18 = 9 * [26 - 34] S18 = 9 * (-8) S18 = -72

So, S18 = -72.