有兩個(gè)數(shù),前三個(gè)數(shù)成等比數(shù)列,后三個(gè)數(shù)成等差數(shù)列,首末兩數(shù)的和是21,中間兩數(shù)的和是18,求這四個(gè)數(shù)?
有兩個(gè)數(shù),前三個(gè)數(shù)成等比數(shù)列,后三個(gè)數(shù)成等差數(shù)列,首末兩數(shù)的和是21,中間兩數(shù)的和是18,求這四個(gè)數(shù)?
數(shù)學(xué)人氣：552 ℃時(shí)間：2020-05-03 17:48:18
優(yōu)質(zhì)解答
設(shè)為x,a-d,a,a+d則x+a+d=21x=21-a-da-d+a=18d=2a-18所以x=21-a-2a+18=39-3a前三個(gè)等比(a-d)²=ax所以(18-a)²=a(39-3a)324-36a+a²=39a-3a²4a²-75a+324=0(4a-27)(a-12)=0a=27/4,a=12a=27/4,d=...
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