4. The sum of third and ninth term of an A.P is 8. Find the sum of the first 11 terms of the progression.
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4. The sum of third and ninth term of an A.P is 8. Find the sum of the first 11 terms of the progression.
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Let a and d are first term and common difference of an AP,
Third term = a + 2d
Ninth term = a + 8d
Sum = 8
a + 2d + a + 8d = 8
2a + 10d = 8 ----( 1 )
Sum of first and 11th terms
= a + a + 10d
= 2a + 10d
= 8 [ from ( 1 ) ]
I hope this helps you.
:)
2a+10d = 8
s11 = 11/2(2a+10d)
= 11/2( 8 )
= 44