Diketahui suatu barisan aritmatika dengan U5= 7 dan U8 = 13. Suku ke 20 adalah... tolong pake cara ya

U8 = a + 7b = 13
U5 = a + 4b = 7
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3b = 6
b =2

a + 4b = 7
a + 8 = 7
a = -1

U20 = a + 19b
= -1 + 38 = 37

Un = a + (n-1)b
U5 = a + (5-1)b
7 = a + 4b

U8 = a + (8-1)b
13 = a + 7b
13 = (a + 4b) + 3b
13 = 7 + 3b
6 = 3b
2 = b

U5 = a + (5-1)(2)
7 = a + 8
-1 = a

U20 = a + (n-1)b
U20 = (-1) + (20-1)(2) = (-1) + 38 = 37