1/1*2+1/2*3+1/3*4+1/4*5= tinh tong nhanh nhat,*la nhan,/ la phan

$\frac{1}{1}$ x 2 + $\frac{1}{2}$ x 3 + $\frac{1}{3}$ x 4 + $\frac{1}{4}$ x 5

= 2 + $\frac{3}{2}$ + $\frac{4}{3}$ + $\frac{5}{4}$ 

= $\frac{24}{12}$ + $\frac{18}{12}$ + $\frac{16}{12}$ + $\frac{15}{12}$ 

= $\frac{24 + 18 + 16 + 15}{12}$ 

= $\frac{73}{12}$