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formula for addition of 1+2+3+4+5................+100 = ?

Answer :

lsv
sum of natural Numbers from 1 to n is given by the formula, 
Sn = n(n + 1)/2 
in your case n = 100 
sum = 100(100 + 1)/2 = 50 × 101 = 5050 

For an A.P. given, formula for its sum is:
[tex]S n=n/2 [a+l] [/tex]
where a = first term of A.P. and l=last term of A.P.
[tex]Sn=100/2 [1+100]=50(101)=5050[/tex]