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Question
Find the sum of all members from 50 to 250 which divisible by 6 and find t13.
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Solution
The numbers between 50 to 250 which are divisible by 6 are
54, 60, 66 ....246
Here a = 54, d = 6 and tn = 246
tn = a + (n - 1) d
246 = 54 + (n - 1) (6)
246 - 54 = 6n - 6
192 = 6n - 6
192 + 6 = 6n
6n = 198
n = `198/6`
n = 33
Sn = `"n"/2 [t_1 + t_n"]`
S33 = `33/2 [54 + 246]`
= `33/2 [300]`
S33 = 4950
tn = a + (n - 1) d
t13 = 54 + (13 -1) (6)
= 54 + 12(6)
= 54 + 72
t13 = 126
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