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Karena U7 = a+6b = 22
a+6(3) = 22
a +18 = 22
a = 22-18
a = 4
Sn = n/2(2a+(n-1)b)
S18 = 18/2 x (2.4+(18-1)3)
= 9 x (8+17.3)
= 9 x (8+51)
= 9 x 59
= 531 (A)
suku- 11 = a + 10b = 34 (2)
eliminasi a dengan (1) - (2) didapat, -4b = -12, jadi b = 3
substitusi b = 3 ke (1) didapat a = 4
S18 = 18/2 . (2(4) + (18 - 1)(3)) = 531 (a)