Question

Find the sum:25+28+31+....+100

Answer 1

a = 25 
d =28 - 25 = 3
an = 100

an = a+( n - 1) d
100 = 25 + (n-1) (3)
100 -25 = (n-1) (3)
75= (n-1) (3)
75/3 = (n-1) 
25 = (n-1) 
25 +1 = n 
26 = n

Sn = n/2 (a +an)
     =  26/ 2 ( 25 +100) 
       = 13 ( 125 )
      = 1625 
       

Answer 2

Answer:

a = 25 

d =28 - 25 = 3

an = 100

an = a+( n - 1) d

100 = 25 + (n-1) (3)

100 -25 = (n-1) (3)

75= (n-1) (3)

75/3 = (n-1) 

25 = (n-1) 

25 +1 = n 

26 = n

Sn = n/2 (a +an)

    =  26/ 2 ( 25 +100) 

      = 13 ( 125 )

     = 1625