Question

obtain the sum of first 48th term of an A.P whose 21st term is 72 &28 term is 156​
obtain the sum of first 48th term of an A.P whose 21st term is 72 &28 term is 156​ ​

Answer 1

Answer:

a+27d=156

a+20d=72

change the sign to minus then you will obtain

7d=84

d=84/7

d=12

put the value of d in any one equation

a+20d=72

a+20(12)=72

a+240=72

a=168

now we can find the sum of 48th term

sum =n/2(2a+(n-1)*d

=48/2(2*168+47*12)

24*900

=21600 is the sum of 48 terms