Question

Find the sum of first 15 term of an A.P. whose n th term is given by an=25-2n

Answer 1

an=25-2n
So,a1=23
     a2=21
     a3=19......
A.P.=23,21,19,17.....................................
s15=15(46+28)/2
       =15*37
        =555 Ans.
If you like it mark it as brainlist............

Answer 2

a↓n= 25-2n      {GIVEN}
 here n=15,

and, a↓1= 25-2(1)
              =25-2
              =23
ATQ, 
S↓n= n/2 (a+a↓n)
               S↓15 = 15/2  [23 +25-2(15)]
                        = 15/2  [23(1+15)]
                        = 15/2  [23 × 16]
                        = 15   [23 × 8]  
                        = 15 (184)
                        =  2760 units

REGARDS

Himanshu1629

<Virtuoso>