Question 936142
Given series is 1,5,9,13,17....
  first term  (a)= 1
 difference b/w first term and second term  = 5-1 =4
difference b/w second and third term  = 9-5=4
every where difference b/w two consecutive terms is 4
hence the give series is Arithmetic series
formula ; {{{ sum = (n/2)(2a+(n-1)d)}}}
  where n= no of terms 
   d is common difference 
   n= 20
d= 4
a = 1
sum = {{{ (20/2)( 2*1+(20-1)*4)}}}
       ={{{ 10*(2+19*4)}}}
          ={{{10*(2+76)}}}
 ={{{10*78}}}
 =780
Result : sum =780