1.diketahui baris geometri u3=3 dan u5=27 tentukan rumus suku ke -n .kemudian hitung jumlah 5 suku pertamaya

Un = ar^n-1

U3 = ar² = 3
U5 = ar⁴
27 = ar² • r²
27 = 3 • r²
r² = 27 ÷ 3
r² = 9
r = √9
r = 3

ar² = 3
a(3)² = 3
a • 9 = 3
a = 3/9
a = ⅓

Sn = a ( r^n - 1 )
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r - 1

= ⅓ ( 3^5 - 1 )
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3 - 1

= ⅓ ( 243 - 1 )
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2

= ⅓ • 242
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2

= 242/ 3 × ½

= 242 × 1/6

= 40 2/6

= 40 ⅓