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Un=a+(n-1)x
U5+U7=(a+4x) + (a+6x) = 2a+10x=144
2a+10x=144 2a+10x=144
a +2x=36 ==> 2a+4x=72
6x=72
x=12, a=12
Jml 10 suku pertama= (10(12+120))/2=660
U₅+U₇ = 144
(1) a + 2b = 36
(2) a + 5b = 72 -
- 3b = - 36
b = 12 ⇔ a = 36 - 2 x 12 = 36 - 24 = 12