The 6th term of an AP is 35 and 13th term is 77 find the sum of the first 20th terms
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tn = a + (n-1) d
t6 = a + 5d = 35.....(1)
t13 = a + 12d = 77........(2)
From 1 and 2 we get d = 6.
Substituting d value in eq....(1) , we get a =5.
Sn= n/2 [2a+(n-+)d]
S20 = 20/2 [(2 × 5) +( 19 × 6)]
S20= 10 × (124)
S20 = 1240
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Answer:
sum of first 20 terms = 1240
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