Tolong Dikerjakan dongg.... Besok mau dikumpul nih

1.a) a=5, b=2,
Un = a+(n-1)b
= 5+(n-1)2
= 5+2n-2
= 2n+3
U20 = 2.20+3
= 40+3 = 43
.b) a=100, b= -10
Un = a+(n-1)b
= 100+(n-1)(-10)
= 100-10n+10
= 110-10n
U20 = 110-10.20
= 110-200
= -90
2. U1+U2+U3 = 15
a+a+b+a+2b = 15
3a+3b = 15
a+b = 5
a = 5-b
U1.U2.U3 = 80
a(a+b)(a+2b) = 80
(5-b).5.(5+b) = 80
(5-b)(5+b) = 16
25-b² = 16
b² = 9
b = 3, a = 5-3 = 2
jadi, a = U1 = 2
a+b = U2 = 5
a+2b = U3 = 2+2.3 = 8
3. U6 = 12
Sn = n/2(a+Un)
S6 = 6/2(a+U6)
12 = 3(a+12)
4 = a+12
a = 4-12 = -8
U6 = a+5b
12 = -8+5b
5b = 20
b = 4
U15 = -8+5.4
= -8+20
= 12