berapakah jumlah dua belas suku pertama deret aritmetika berikut a.-5+(-2)+1+... b.6+1+(-4)+... c.32+16+8+...

A. -5 + (-2) + 1 + ...
a = -5 dan b = 3
Sn = 1/2n [2a + (n-1)b]
S11 = 1/2(11) [2(-5) + 10(3)]
S11 = 11/2 (-10 + 30)
S11 = 11/2 (20)
S11 = 11(10)
S11 = 110

B. 6 + 1 + (-4) + ...
a = 6 dan b = -5
Sn = 1/2n [2a + (n-1)b]
S11 = 1/2(11) [2(6) + 10(-5)]
S11 = 11/2 (12 - 50)
S11 = 11/2 (-38)
S11 = 11(-19)
S11 = -209

C. 32 + 16 + 8 + ...
a = 32 dan b = -16
Sn = 1/2n [2a + (n-1)b]
S11 = 1/2(11) [2(32) + 10(-16)]
S11 = 11/2 (64 - 160)
S11 = 11/2 (-96)
S11 = 11(-48)
S11 = -528