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01-08-2016
Mathematics

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The third term of an arithmetic sequence is 21, and the eighth term is 56. The first term is

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01-08-2016

[tex]a_3=21;\ a_8=56\\\\a_8-a_3=5d\\\\5d=56-21\\5d=35\ \ \ |divide\ both\ sides\ by\ 5\\d=7\\\\a_1=a_3-2d\\\\a_1=21-2\cdot7\\a_1=21-14\\a_1=7\\\\Answer:\boxed{a_1=7}[/tex]

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