Răspuns:
Explicație pas cu pas:
1+ 1/1+2 +1/1+2+3 +1/1+2+3+4 +....+1/1+2+3+...+x= (folosim sume Gauss)
1+1/2·3/2+1/3·4/2+...+1/x(x+1)/2=
1+2/2·3+2/3·4+.....+2/x(x+1)+1-1=
2[1+1/2·3+1/3·4+...+1/x(x+1)]-1=
1/2·3=1/2-1/3 folosim formula 1/x(x+1)=1/x-1/(x+1)
1/3·4=1/3-1/4
1/(x-1)x=1/(x-1)-1/x
1/x(x+1)=1/x-1/(x+1)
.............................
2+ 1-1/(x+1)-1=400/201 ⇒2x/(x+1)=400/201 x/(x+1)=200/201
201x=200x+200 ⇒x=200