Question

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

Answer 1

a=4
an=81
an=a+(n-1) d
81=4+(n-1)7
81-4=(n-1)7
77=(n-1)7
77/7=n-1
11=n-1
n=12
Sn=n/2(2a+(n-1)d)
12/2(2*4+(12-1)7)
6(8+77)
6(85)
510 Ans

Answer 2

so the no of terms is 12 and their sum is 510