If t4 =22 and t7=37 find Sn and S20 of an A.P
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Answer:
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Step-by-step explanation:
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Answered by
1
Answer:
Sn = n/2{8 + 5n}
S20 = 1090
Step-by-step explanation:
Given :-
t4 = a + 3d = 22
t7 = a + 6d = 37
Sn = ?
S20 = ?
Subtracting t4 from t7,
a + 6d = 37
- a + 3d = 22
3d = 15
d = 5
Now putting d in t4,
a + 15 = 22
a = 7
We know that
Sn = n/2{2a + d(n - 1)}
= n/2{14 + 5(n - 1)}
= n/2{14 + 5n -5}
= n/2{8 + 5n}
now,
Sn = n/2{2a + d(n - 1)}
S20 = 20/2{14 + 5(19)}
= 10{14 + 95}
= 1090
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