Tentukan jumlah n suku pertama dari masing masing deret arimetika berikut a. n=18, a=-3, dan u18= 31 B. N= 9, a=6, dan u9= -26

A. U18 = a+(n1)b
U18 = -3+(18-1)b = 31
U18 = (18-1)b = 31+3
U18 = 17b = 34
U18 = b = 34/17 = 2

S18 = n/2 (2a+(n-1)b)
S18 = 18/2 (2(-3) + (18-1)2)
S18 = 9 (-6+34)
S18 = 9(28) =252

B. U9 = 6+(9-1)b = -26
U9 = 8b = -26-6
U9 = b = -32/8 = -4

S9 = 9/2 (2(6)+(9-1)-4)
S9 = 9/2 ( 12-32)
S9 = 9/2 ( -20)
S9 = 9 x -10 = -90