18. Find the sum of 5 + 8 + 11 + ... to 10 terms using the formula,
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Given AP: 5+ 8+ 11+........10 terms
here, a = 5 , n= 10 , d =3
10th term = a + (n - 1) d
= 5 + (10 -1) 3
= 5+ 9 (3)
=5+27
therefore, 10th term=32
sum of 10 terms = n/2 ( a+ 10th term )
=10/2 ( 5+32)
= 5 ( 37 )
=185
therefore, sum of the given terms is 185
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