Answer :

apologiabiology
summaation of a sequence

or see a smaller scale
from 1 to 10
notice
1+10=11
2+9=11
etc
5+6=11
10/2=5
there are 5 pairs
5*11=55

so

1 to 100
1+100=101
2+99=101
...
50+51=101

100/2=50
there ar 50 pairs
50 times 101=5050

the sum is 5050

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