A progression has a second term of 96 and a fourth term of 54. Find the first term of the progression in each of the following cases:
(i) the progression is arithmetic,
(ii) the progression is geometric with a positive common ratio.
Solution
(i) For an arithmetic progression, the second term is given by:
\(a + d = 96\)
and the fourth term is:
\(a + 3d = 54\)
Subtract the first equation from the second:
\((a + 3d) - (a + d) = 54 - 96\)
\(2d = -42\)
\(d = -21\)
Substitute \(d = -21\) back into \(a + d = 96\):
\(a - 21 = 96\)
\(a = 117\)
(ii) For a geometric progression, the second term is:
\(ar = 96\)
and the fourth term is:
\(ar^3 = 54\)
Divide the second equation by the first:
\(\frac{ar^3}{ar} = \frac{54}{96}\)
\(r^2 = \frac{9}{16}\)
\(r = \frac{3}{4}\)
Substitute \(r = \frac{3}{4}\) back into \(ar = 96\):
\(a \times \frac{3}{4} = 96\)
\(a = 128\)
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