Question

1+2+3+....+n=n(n+1)/2

Answer 1

let

s=1+2+3+..............+n

now write the reverse order.

s=n+(n-1)+(n-2)+.............3+2+1

Add both the equations, we get

2s=(n+1)+(n+1)+(n+1)+..........................n times

so,

2s=n (n+1)

s=n (n+1 )2

:-)hope it helps u.

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