pliiss kak tolong bantuan jawaban + cara

1.
U3 + U12 = 46
a + 2b + a + 11b = 46
2a + 13b = 46

U21 = 77
a + 20b = 77

Eliminasi
2a + 13b = 46  (x1)
a + 20b = 77    (x2)
---------------------------
2a + 13b = 46
2a + 40b = 154
-------------------------- -
-27b = -108
b = (-108) / (-27)
b = 4

a + 20b = 77
a + 20(4) = 77
a + 80 = 77
a = 77 - 80
a = -3

U51 = a + 50b
= -3 + 50(4)
= -3 + 200
= 197

2.
U3 = 5
a + 2b = 5

U12 = 41
a + 11b = 41

eliminasi
a + 2b = 5
a + 11b = 41
---------------------- -
-9b = -36
b = 4

a + 2b = 5
a + 2(4) = 5
a + 8 = 5
a = -3

Sn = n/2[2a + (n-1)b]
S20 = 20/2 [2(-3) + (20-1)4]
S20 = 10 (-6 + (19 x 4))
S20 = 10 (-6 + 76)
S20 = 10 x 70
S20 = 700