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Question
Find the sum of the following APs:
2, 7, 12, ..., to 10 terms.
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Solution
2, 7, 12, …, to 10 terms
For this A.P.,
a = 2
d = a2 − a1
d = 7 − 2
d = 5
n = 10
We know that,
Sn = `n/2 [2a + (n - 1) d]`
S10 = `10/2 [2(2) + (10 - 1) × 5]`
= 5[4 + (9) × (5)]
= 5 × 49
= 245
Thus, the sum of first 10 terms is 245.
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1,3,5,....,149 .
Here a = 1 , d =b`[ ], t_n = 149`
tn = a + (n-1) d
∴ 149 =`[ ] ∴149 = 2n - [ ]`
∴ n =`[ ]`
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