cara mengerjakannya gimana kak. tolong bantuan

jawaban no 2
S20 = 700





jawaban no 1
u3 + u12 =46
{a + (3-1)b} + {a+(12-1)b} = 46
a + 2b + a + 11b = 46
2a + 13b = 46.....(1)

u21 = a + (21-1)b
77 = a + 20b .....(2)

pers (1) dan(2)
2a + 3b = 46 | 1 |
a + 20b = 77 | 2 |
_____________×
2a + 3b = 46
2a + 40b = 154
_____________-
-27b = -108
b = -108/-27=4

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

u51 = a + (51-1)b
= -3 + 50(4)
= -3 +200
= 197