The sum of the first 5 terms of an arithmetic progression is 30.
The sum of the sixth to tenth terms of the same progression is 80.
What is the sum of the
to
terms?
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The formula for the sum of terms of an arithmetic progression is s = 2 n ( a 1 + a n )
Considering the first five terms, we have
3 0 = 2 5 ( a 1 + a 5 )
After simplifying, it becomes
a 1 + a 5 = 1 2
But we know that
a 5 = a 1 + 4 d
After substituting and simplifying, we get
2 a 1 + 4 d = 1 2 ( 1 )
Considering the first 1 0 terms, we have
s ( 1 − 1 0 ) = 3 0 + 8 0 = 1 1 0
1 1 0 = 2 1 0 ( a 1 + a 1 0 )
After simplifying, it becomes
a 1 + a 1 0 = 2 2
But we know that
a 1 0 = a 1 + 9 d
After substituting and simplifying, we get
2 a 1 + 9 d = 2 2 ( 2 )
Subtract ( 1 ) from ( 2 ) , we get
5 d = 1 0
d = 2
Solving for a 1 by substituting 2 for d in ( 1 ) or ( 2 ) , we get
2 a 1 + 4 d = 1 2 ⟹ 2 a 1 + 4 ( 2 ) = 1 2 ⟹ a 1 = 2
or
2 a 1 + 9 d = 2 2 ⟹ 2 a 1 + 9 ( 2 ) = 2 2 ⟹ a 1 = 2
Solving for a 1 0 0 and a 1 5 0 , we have
a 1 0 0 = a 1 + 9 9 d = 2 + 9 9 ( 2 ) = 2 0 0
a 1 5 0 = a 1 + 1 4 9 d = 2 + 1 4 9 ( 2 ) = 3 0 0
Solving for the sum of a 1 0 0 to a 1 5 0 , we have
s ( 1 0 0 − 1 5 0 ) = 2 5 1 ( 2 0 0 + 3 0 0 ) = 1 2 7 5 0