1/2+1/6+1/12+1/20+1/30+1/42 +.+1/90
=1-1/2+1/(2*3)+1/(3*4)+1/(4*5) +.+1/(9*10)
=1-1/2+1/2-1/3+1/3-1/4+...+1/9-1/10
=1-1/10
=9/10