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= a + 4b
U8 = a + (n-1)b
= a +7b
U5 +U8 = a + 4b + a + 7b
29 = 2a + 11b ......... (i)
U3 = 18
a + 2b = 18 ....... (ii)
eliminasikan persamaan (i) dan (ii)
2a + 11b = 29 | x 1| 2a + 11b = 29
a + 2b = 18 | x 2| 2a + 4b = 36
----------------------- -
7b = -7
b = -1
subtitusikan nilai b ke pers. (ii)
a + 2b = 18
a - 2 = 18
a = 20
Jadi, U31 = a + (n-1)b
= 20 + 29 (-1)
= 20 - 29
= -9