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Find the sum of the A.P. $7, 10\frac{1}{2}, 14, \dots, 84$.
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$a = 7, d = \frac{21}{2} - 7 = \frac{7}{2}$ [$1$ mark]
$84 = 7 + (n - 1) \times \frac{7}{2} \Rightarrow n = 23$ [$1$ mark]
$S_{23} = \frac{23}{2}(7 + 84) = \frac{2093}{2}$ [$1$ mark]
$84 = 7 + (n - 1) \times \frac{7}{2} \Rightarrow n = 23$ [$1$ mark]
$S_{23} = \frac{23}{2}(7 + 84) = \frac{2093}{2}$ [$1$ mark]