Question

sum of all natural numbers less than100

Answer 1

sum of all natural nos less than 100 is 1+2+3+.............+99.here a= 1;d=1; l=99.
this is an AP
n=l-a/d +1=99-1/1 +1=99
sum = n/2(a+l)=99/2(1+99)=99/2(100)=50×99=4950.
therefore the sum is 4950.

Answer 2

Less than 100 means till 99., 1,2,,3,4,5,6,7,8,9,10,........99
We have the formula for this type of sums in the topic Arithmetic Progression.
Sum = n/2[2a + (n-1)d]
where n = total number of terms
a= first term
d= difference between two consecutive terms(2-1=1, 3-2=1, 4-3=1, 5-4=1......)

Now, we have all required value to find the sum of all natural numbers less than 100, 
a=1
n=99
d=1
Putting all values in the formula Sum = n/2[2a + (n-1)d]
we get 4950 as the answer..