Question

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

Answer 1

use the formulae an=a+(n-1) ×d
an=81
a=4
d=7
81=4+(7n-7)
81=-3+7n
81+3=7n
84=7n
n=84÷7
$\frac{84}{7 } = answer = n$

Answer 2

Tn=a+(n-1)d :

81=4+(n-1)7 :

81-4=7n-7 :

77+7=7n :

84=7n :

n=84/7 :

n = 12




There are 12 term in an ap


S12=12/2[2a+(n-1)d]