It is given that the first 51 terms of an A.P whose second and third terms are 14 and 18 respectively. Find the sum.
Arithmetic Progression chapter 9 question no 9 (ML Aggarwal, Avichal Publication)
Rajan@2021Guru
Find the sum of first 51 terms of an A.P whose second and third terms are 14 and 18 respectively.
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a2=14 and a3=18
Common difference, d=a3−a2=18−14=4
Now
a2=a+d=14
a+4=14
a=10
Now, sum of 51 terms
Sn=2n(2a+(n−1)d)
S51=251(2(10)+(51−1)4)
=251(20+200)
=251×220
=51×110=5610
S51=5610