tolong yang menjawab harus janji pakai cara

Un = a + (n-1)b

U2 + U5 = 1
(a + b )+(a + 4b) = 1
2a + 5b = 1

U4 + U7 = 21
(a + 3b)+(a + 6b) = 21
2a + 9b = 21

eliminasi
2a + 5b = 1
2a + 9b = 21
-------------------- -
         -4b = - 20
            b = 5

2a + 5b = 1
2a + 5(5) = 1
2a + 25 = 1
2a = 1 - 25
2a = -24
a = -12

a) U15 = a + 14b = -12 + 14(5) = -12 + 70 = 58
b) Sn = n/2 (2a + (n-1)b)
    S12 = 12/2 (2(-12) + (12-1)5)
    S12 = 6 (-24 + (11 x 5))
    S12 = 6 ( -24 + 55)
    S12 = 6 x 31
    S12 = 186