1／3+1／6+1／10+1／15+1／21+1／28+1／36+1／45
=2*(1/6+1/12+1/20+1/30+1/42+...+1/90)
=2*(1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+...+1/9*10)
=2*(1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+...+1/9-1/10)
=2*(1/2-1/10)
=2*2/5
=4/5