1/1 +1/(1+2)+1/(1+2+3) +....+1/(1+2+3+...+x)=200/101
1/1+1/3+1/6+1/10+1/15+....2/x*(x+1)=200/101
2/1*2 +2/2*3+2/3*4+2/4*5+.....2/x*(x+1)=200/101
impartim prin 2 toata relatia
1/1*2+1/2*3+1/3*4+1/4*5+...+1/x(x+1)=100/101
1-1/2+1/2-1/3+1/4-1/5+.....+1/x-1/(x+1)=100/101
1-1/(x+1)=100/101
(x+1-1)/(x+1)=100/101
x/(x+1)=100/101
101x=100x+100
x=100
Obs ..am tinut cont ca 1+2+...+x= x(x+1)/2
deci 1/(1+2+...+x)=2/(x*(x+1))
