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Verified answer
Un = a × r^(n-1)U2 = a × r
21 = a × r
a = 21/r
U4 = a × r^3
189 = a × r^3
a = 189/r^3
21/r = 189/r^3
r^2 = 9
r = 3
rasio = 3
a × r = 21
a × 3 = 21
a = 7
suku pertama = 7