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Arithmetic Series or Sum of Arithmetic Sequence:

     S _{n} =  \frac{n}{2}  ( a_{1}+a _{n})


      S_{n} = sum of the given arithmetic sequence

     n = number of terms ⇒  10,000

     a₁ = first term in the sequence  ⇒  1

      a_{n} = last sequence/last term value  ⇒  10,000


      S_{10,000}=  \frac{10,000}{2}(1 + 10,000)

      S_{10,000} = 5,000 (10,001)

      S_{10,000} = 50,005,000

ANSWER:  50,005,000

You can use this formula
                                              n(n+1)/2       /  n  = the last term
                                     thus  (10,000 x 10001)/2
                                            =   5000 x 10,001
                                            =   50,005,000
Hi, the equation you use is only true and applicable if the first term is 1, that is, 1, 2, 3, ... Hence, we can not say that it is the formula for the arithmetic series or sum of arithmetic sequence. Thanks :-)
what you say is correct, this formula can only be used if
what you say is correct,this formula is applicable only if the first term is one
Thanks. Please keep on posting answers. :-)