中五等差數列(高難度一問)
回答 (1)
2009-09-11 5:33 am
✔ 最佳答案
7th term is a + 6d.11th term is a + 10d
so 7(a + 6d) = 11(a + 10d)
7a + 42d = 11a + 110d
4a + 68d = 0
a + 17d = 0 which is the 18th term.
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