Question

the first and the last terms of an AP are 4 and 81 respectively. if the common difference is 7 , how many terms are thero in the AP and what is their sum

Answer 1

formula of ap: an=a+(n-1)d
81=4+(n-1)7
77=7(n-1)
11=n-1
so n=12
so there are 12 terms
sum of ap=n/2× ( a+l)
so sum=12/2×(81+4)
=6×85=510
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