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3. 設〈〉為等比數列, az + as = 84 且a2 + as = 12,求首項a?「ob
15
a₁xr5 = 84
aixr²³ = 12
A1 =
a₁xv + a₁xr4 = 84
aixr²
2
aixr + aixr² = 12
r² = 7= r = ²√7
12x√7
7√7-√7
1 = √7 (1)
:49 15

Answer 1

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