Question

given d=5 s9=75 find a and a9

Answer 1

Hope u like my process
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Formula to be used
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Sn =
$\frac{n}{2} (2a + (n - 1)d)$
An = a + (n-1)d
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d = 5

S9 = 75
$= \frac{9}{2} (2 a + (9- 1)5) \\ \\ or. \: \: 75 = \frac{9}{2} (2a + 8 \times 5) \\ \\ or. \: \: 75 = \frac{9}{2} \times 2 (a + 20) \\ \\ or. \: \: 75 = 9a + 180 \\ \\ or. \: \: 9a = 75 - 180 \\ \\ or. \: \: a = - \frac{105}{9} = - \frac{35}{3} \\ \\ or. \: \: a = - 11.66$

A9 = a +( n-1)d

$\: \: \: \: = - \frac{35}{3} + (9 - 1)5 \\ \\ \: \: \: \: = - \frac{35}{3} + 40 \\ \\ \: \: \: \: = \frac{120 - 35}{3} = \frac{85}{3}$

Hope this is ur required answer
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