Question

prove that 1+2+......+n= n(n+1)/2​

Answer 1

suppose, S=1+2+3+............................. n (term)

Also S=n+(n-1)+(n-2)..........................3+2+1

adding that

2S=(n+1)+(n+1)+(n+1)+(n+1)+(n+1)+(n+1)+...........................(n+1)

2S =n.(n+1)

S=n.(n+1)/2

Hence proved.

Answer 2

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

1+ 2 = -3