Sum of odd numbers

Find the sum of all odd integers from 13 to 781.

Correct answer:

s =  152845

Step-by-step explanation:

a1=13 d=2  b=781  b=a1+d⋅ (n−1)  n=(b−a1)/d+1=(781−13)/2+1=385  s=n⋅ a1+b2=385⋅ 13+7812=152845

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