tolong yg bisa jawab 1 , 3, 4, 5

No. 1
adalah barisan geometri dengan 
a = 3
dan r = 1/3

Un = a r^(n-1)
U8 = a r^(7)
U8 = 3 . (1/3)^7
U8 = (1/3)^6
      = 1/729


No.3
a = 20
U10 = a + 9b = 47
           20 + 9b = 47
                   9b = 27
                     b = 3
Sn = n/2 (2a + (n-1)b)
S12 = 12/2 (2(20) - 11(3))
S12 = 6 (40 + 33)
       = 6(73)
       = 438


No. 4
Sn = 2n²(n - 1)
S1 = 0 = a
S5 = 2(25)(5 - 1)
     = 50(4)
     = 200

Sn = n/2 [a + Un ]
S5 = 5/2 [0 + U5]
200 = 5/2 (U5)
U5 = 400/5
U5 = 80

No. 5
a = 4
U5 = a r^4 = 324
      = r^4 = 324/4
      = r^4 = 81
         r = 3

S6 = 4(3^6  - 1) / (3 - 1)
      = 4(729 - 1) / 2
      = 4(728) / 2
      = 4(364)
      = 1456