Tentukan lima suku pertama dari barisan aritmetika jika diket: U9 =20 dan b=5 U23= -3 dan b=-3 Please tolong bantu yach!!

U9 = 20 , b = 5

U9 = a+(9-1)b
20 = a+(8)5
20 = a + 40
a = -20

S5 = 5/2 (2(-20) + (5-1)5)
S5 = 5/2 (-40+20)
S5 = 5/2 (-20)
S5 = -50

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U23 = -3, b = -3

U23 = a + (23-1)b
-3 = a + (22)-3
-3 = a -66
a = 63

S5 = 5/2 (2(63) + (5-1)-3)
S5 = 5/2 (126-12)
S5 = 5/2 (114)
S5 = 285