I need help proving this statement. Any help would be great!
Answer
Here is an approach.
sn=1+2+3+⋯+(n−1)+nsn=n+(n−1)+(n−2)+⋯+1.
Adding the above gives
2sn=(1+n)+(2+(n−1))+(3+(n−2))+⋯+(1+n)
=(1+n)+(1+n)+⋯+(1+n)
The above is nothing but adding (1+n) n times and the result follows
⟹sn=n(n+1)2.
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