dik: U3: 66 U5: 108 n:12dit: a. U1 b. Sn

Dik. U3 = 66, U5 = 108 & n : 12
Dit. U1(a) & Sn ?

Jawab...
U3 = a + 2b = 66
U5 = a + 4b = 108
-------------------- -
@ -2b = -42
@ b = 21

a (U1) :
U3 = a + 2b = 66
@ a + 2(21) = 66
@ a + 42 = 66
@ a = 66-42
@ a = 24

Sn = n/2 (2a+(n-1) b)
S12 = 12/2 (2(24) + (11) 21)
S12 = 6 (48 + 231)
S12 = 6 ( 279)
S12 = 1962

Semoga membantu.. ^_^