Найдите а1, если :
1)d=20; S6=60
2)d=-40; S9=1350
Найдите b1 и bn геометрической прогрессии, если

1. 1) S6 = (( a1+ 5d)/2 ) * 6 = 60.

(a1+5d) = 20

a1 = 20 - 5d = 20 -100 = -80.

2) the same method:

S9 = ((a1 + 8d)/ 2) * 9 = 1350.

a1+8d = 75

a1 = 75 - 8d = 75 - (-40*8)= 75 + 320 = 395.

1. 1) b3 = b1 * q^2.

b1 = b3 / q^2 = 81 / (-1/3 ^2) = 81 / (1/9) = 81 * 9 = 729.

bn (n=8), b8 = b1 * q^7 = b3 * q^5 = 81 * (-1/3 ^5) = 81 * (-1/243) = - 1/3

2) the same method:

b1  = b5 / q^4= 64 / (-1/2^4) = 64 / (1/16) = 64 * 16 = 1024

bn (n=9), b9 = b5* q^4 = 64 * (-1/2^4) = 64 * 1/16 = 64/16 = 4

Объяснение: