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b=2
Un=a+(n-1)b
225=1+(n-1)2
225=n-2
227=n
.
no.2
U_n = 2n-1
1+3+5+...+(U_n) = n^2
n^2=225
n = 15
jadi
U_n atau U_15 = 2(15)-1 = 29