In an AP
Given d = 5, S9 = 75, find a and a9.
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Solution
Given that, d = 5, S9 = 75
`S_n = n/2[2a+(n-1)d]`
`S_9 = 9/2[2a+(9-1)5]`
75=9/2(2a+40)
25 = 3(a + 20)
25 = 3a + 60
3a = 25 − 60
`a = (-35)/3`
an = a + (n − 1)d
a9 = a + (9 − 1) (5)
`= (-35)/3+8(5)`
`=(-35+120)/3=85/3`
Concept: Sum of First n Terms of an AP
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