salut !!!
wach hadchi
ghir lcours alors:
Sn=U0 + U1 + ....+U(n)
Sn= U(n) + ........+ U(0) (ljam3 tabadoli)
2S(n)=(u(0)+u(n)) + (u(1) + u(n-1)) +....+(u(n)+u(0))
= (U(0) + U(n))(n+1)
====> S(n) = (u(0) + u(n))(n+1)/2
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mmm