Question

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

Answer 1

Here, a= 4, an = 81, d = 7

an = a + (n-1) d

=> 81 = 4 + (n-1) 7

=> 81 = 4 + 7n - 7

=> 7n = 81 + 3

=> 7n = 84

=> n = 12

There are 12 terms in the given AP

Sn = n/2(a + an)

Sn = 12/2( 4 + 81)

Sn = 6(85)

Sn = 510

#BAL#answerwithquality

Answer 2

Answer:

Step-by-step explanation:an=a+(n-1)d

81=4+(n-1)7

77=(n-1)7

11=n-1

N=12

Sn=6(4+81)

6*85

520