The first 100 natural numbers is an arithmetic sequence where the first term a_1=1, 100th term a_{100}=100 and the common difference d=1

Using the sum formula for an arithmetic sequence,

S_n=\dfrac{a_1+a_n}{2}\cdotp d\cdotp n

S_{100}=\dfrac{1+100}{2}\cdotp 1\cdotp 100

=101\times 50

=5050