(a)+(a+d)+(a+2d)=(a)(a+d)(a+2d)
Express a in terms of d
arithmetic sequence
2009-11-08 1:02 am
回答 (1)
2009-11-08 1:09 am
✔ 最佳答案
(a)+(a+d)+(a+2d)=(a)(a+d)(a+2d)3a + 3d = a(a+d)(a+2d)
3(a+d) = a(a+d)(a+2d)
when a + d = 0
a = - d
when a + d is not = 0 :
3 = a(a+2d)
3 = a^2 + 2ad
a^2 + 2da - 3 = 0
a = {-2d +/- √[4d^2 - 4(-3)]} / 2
a = - d +/- √(d^2 + 3)
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https://hk.answers.yahoo.com/question/index?qid=20091107000051KK01023