2/3+2/15+2/35+2/63+2/99
=2*(1/3+1/15+1/35+1/63+1/99)
=2*(1/3+1/3*5+1/5*7+1/7*9+1/9*11)
=2*[1/2*(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)
=1-1/11
=10/11
2/3+2/15+2/35+2/63+2/99
=2*(1/3+1/15+1/35+1/63+1/99)
=2*(1/3+1/3*5+1/5*7+1/7*9+1/9*11)
=2*[1/2*(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)
=1-1/11
=10/11