We have First odd number is 1 Last odd number is 49 a = 1 an or I = 49 ,common difference d= 2. an = a+(n-1)d =1+(n-1)2 = 49 =1+2n-2 =49 =2n-1 =49 =2n=49+1 =2n=50 n=25 sn=n/2(1+49) =25/2(1+49) =25/2* 50 25 *25 = 625 therefore sum of all odd numbers between 0 to 50 is 625.