Question

wht will be the sum when 1+3+5+7...........249

Answer 1

a(first term)=1 and L(Last term)=249,d(difference)= 2nd term-1st term
2nd term is 3 and 1st term is 1 so 3-1=2, common difference=2
to find n's value using formula a+(n-1)d=249(last term)
putting the value of a and d : 1+(n-1)2=249
1+2n-2=249
-1+2n=249
2n=249+1
2n=250
n=125
now sum of series=n/2[a+L]
putting the value of n,a,L
then 125/2[1+249]
125/2[250]
125*125=15625
                                 

Answer 2

given, 
1+3+5+7+............249
here the given list of numbers are in a.p.so the sum of n numbers of an a.p is
sn=n/2(a+l)...............(1)
here,
sn is the sum of the given list of numbers,
n is the no. of numbers in the given list,
a is the 1st number in the list,
l is the last number in the list. 
here,to find n
we  know that
an=a+(n-1)d
249=1+(n-1)2
249=1+2n-2
n=125

sub this value in (1)
sn=n/2(a+l)
   =125/2(1+249)
   =125/2x250
   =15625
hope this helped u.......:))