185. а1=103, d = -2
а) S(n) = (2a1+d(n-1))*n/2. Тогда:
S(8) = (206 - 14)*8/2 = 768
б) S(103) = (206 - 204)*103/2 = 103
186.
а)А₁=7,d=4, n=13;
a(n) = a(1)+d(n-1) = 7+4n-4 = 4n+3 = 55
S(n) = (14+4(n-1))*n/2 = 403
б)А₁=2,d=2,n=40;
A(n) = 2+2*39 = 80;
S(n) = (4+2*39)*40/2 = 1640
в)A₁=56,d=-3,n=11
A(n) = 56 - 3*10 = 26
S(n) = (112-3*10)*11/2= 451
188. Y1= -32, d = 5
a) S(10) = (-64 + 5*9)*10/2 = -95
б) S(26) = (-64 + 5*25)*26/2 = 793
189. a1 = 25, d = -4,5
a) S(16) = (50-4,5*15)*16/2 = - 140
б) S(40) = (50 - 4,5*39)*40/2 = - 2510
1)
(83a-91b)-(89a-100b) =
= 83a-91b - 89a+100b =
= (83a-89a)+(100b-91b) =
= - 6a + 9b = - 3(2a - 3b)
2)
(5k+6t)+(2.8t-3.1k) =
= 5k+6t + 2,8t - 3,1k =
= (5k-3,1k)+(6t+2,8t) =
= 1,9k + 8,8t