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25 September, 22:07

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

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  1. 25 September, 23:24
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    This is the concept of sequences and series

    the nth term of the arithmetic sequence is given by:

    nth=ar^ (n-1)

    where;

    a=first term

    r=common ratio

    n=th number of terms;

    thus;

    21=ar^ (3-1)

    21=ar^2 ... i

    the eighth term will be:

    56=ar^ (8-1)

    56=ar^7 ... ii

    thus from i

    a=21/r^2

    from ii

    a=56/r^7

    hence combining i and ii we get:

    21/r^2=56/r^7

    multiplying through by a^7 we get:

    21r^5=56

    dividing both sides by 21 we get:

    r^5=8/3

    hence;

    r = (8/3) ^ (1/5) = 1.22

    thus, the value of a will be:

    a=21/r^2

    a=21 / (1.22) ^2

    a=14.185

    the first term is 14.185
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