Răspuns:
Explicație pas cu pas:
a=1/1x2+1/2x3+.......+1/42x43
1/1x2=1/1-1/2
1/2x3=1/2-1/3
.................
1/42x43=1/42-1/43
a=1-1/43=42/43
b=1/2x3+1/3x4+.......+1/125x126
1/2x3=1/2-1/3
1/3x4=1/3-1/4
........................
1/125x126=1/25-1/126
b=1/2-1/126=(63-1)/2x126=62/126=31/43
a/b=42/43×43/31=42/31
