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a = 1/27
U4 = ar³
= 1/27 . 3³
= 1/27 . 27
= 1
U7 = a . r^6
= 1/27 . 3^6
= 1/3³ . 3^6
= 3³
= 27
U4 + U7 = 1 + 27 = 28