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In an AP Given d = 5, S9 = 75, find a and a9. - Mathematics

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`

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APPEARS IN

NCERT Class 10 Maths
Chapter 5 Arithmetic Progressions
Exercise 5.3 | Q 3.05 | Page 112
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