bantu jawab guys pake cara a,b,c aja please bantu jawab

a.)
a = 2 ; b = 3
Un = a + (n - 1)b
U10 = 2 + (10 - 1)3
U10 = 2 + 27
U10 = 29

Sn = n/2 (a + Un)
S10 = 10/2 (2 + 29)
S10 = 5 . 31
S10 = 155

b.)
a = 15 ; b = -2

Sn = n/2 (2a + (n - 1)b)
S12 = 12/2 (2.15 + (12 -1)(-2))
S12 = 6 (30 + 11.(-2))
S12 = 6 (30 + (-22))
S12 = 6 . 8
S12 = 48

c.)
a = -8 ; b = 3

Un = a + (n - 1)b
U9 = -8 + 8.3
U9 = -8 + 24
U9 = 16

Sn = n/2 (a + Un)
S9 = 9/2 (-8 + 16)
S9 = 9/2 . 8
S9 = 36

Semoga membantu dan semoga bermanfaat...