A1 = - 81, a2 = - 54, a3 = - 36, a4 = - 24,... The sequence is geometric sequence. r = a2/a1 = (-54)/(-81) = 2/3 The common rate of the sequence is 2/3. The sum of the first 5 terms: S 5 = a1* (1-(2/3)^5) / (1-2/3) = = 81 * ( 1- 32/243)/ (1/3) = = 81 * ( 211*81 ) = 211
We have the following remark -81/-54= -54/-36 = -36/ -24=...= 1.5 the sequence is geometric sequence the common ratio of the sequence is 1.5 the sum of the five terms of the sequence is S5= a[1- (1.5)^5 / 1-1.5], a first term, S5= -42*6.5= - 276
