1+1／3+1／6+1／10+1／15+1／21+1／28
=1+2*(1/6+1/12+1/20+1/30+1/42+1/56)
=1+2*(1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+1/7*8)
=1+2*(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8)
=1+2*(1/2-1/8)
=1+2*3/8
=1+3/4=7/4