given d=5 , s9=75 , find 'a'
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d = 5
S9 = 75
By applying the formula
Sn = n / 2 + ( 2a + ( n - 1 ) × d
75 = 9 / 2 + (2a + ( 9 - 1 ) × 5
75 = 9 / 2 + ( 2a + 40 )
75 × 2 / 9 = 2a + 40
50 / 3 = 2a + 40
50 / 3 - 40 / 1 = 2a
by taking LCM we will get
- 70 / 3 = 2a
a = - 70 / 3 × 2
a = - 35 / 3.
a9 = a + 8d
= -35 / 3 + 8 × 5
= - 35 / 3 + 40
= -35 + 120 / 3
= 95 / 3.
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