Question

in an Ap the sum of first n terms is 3nsquare/2 +5n/2 find its 25 term

Answer 1

Given
         (3n^2/2)+(5n/2)
so
  n=1
         (3n^2/2)+(5n/2)
=(3(1)^2/2)+(5(1)/2)  
=  (3/2)+(5/2)
=(3+5)/2
=4 =a        (since S(1) is a)

n=2
(3n^2/2)+(5n/2)
=(3(2)^2/2)+(5(2)/2)
=(3(4)/2)+(10/2)
=(12/2)+(10/2)
 = 6+5
=11 = a+d          (since S2 is a+d)
so
 a=4
a+d=11
4+d=11
d=7
n=25
 
a(n)=a+(n-1)d
a(25)=4+(25-1)7
        =4+(24)7
        =4+168
   a(25)=172
thumbs up plz